What is a deadhead and what does s/he do?

A dead head is an airline crew member, a pilot or a cabin crew, who is assigned to fly to a particular destination to assume a duty. In the flight where he or she is in, she's not supposed to work as a crew. He or she shall be in a complete uniform, thus making him or her able to sit on a jumpseat if the flight is full. Deadheading crew are also paid based on their flying time.

But I don't understand. What exactly does s/he do? Nothing? Just sit on the plane? Why are they paid then? What is the said 'to assume a duty'? So the airline company pays them to fly to a particular destination to do something there? Can someone explain please

Yes, the airline pays them to travel to a place where they will work. They are paid because the airline is using up their time, even if they don't do any actual work while they are travelling.

Nothing. That person is just being transported. For instance, a pilot might be transported to an airport to pick up an airplane and fly passengers somewhere else.

Airlines need to match crews and flights. A flight crew has work hours limitations. The airline sometimes needs to fly in crew members and they are on business travel with some amount of pay for their time not actually working.
We have seen passengers get bumped to get a crew to a city and airlines do not want to do it, but have to do so sometimes.
If in uniform, they can use an extra crew seat on the plane rather than bumping a passenger.

A business can need an employee at another location to do a job. I did it many times in my career. Business travel can be intracompany and this flight crew movement is no different.

Deadhead is nothing more than a pilot or FA who is repositioning as part of an on-duty assignment. They are working but not part of that flight's assigned crew

Not complicated.
A term that has been in use for decades in the transportation industry.
In this situation it is an employee being transported to someplace where they are needed.
They are not expected to be part of the working crew on the flight.

They are being paid to to go to some place where they are needed.

A deadhead flight is an empty plane that is dispatched to some place. It might just have the pilots or could have a full crew.
An example of this is a "rescue" flight to fetch stranded passengers. Could be a plane is out of service at the place or some disaster and they need an extra plane to do something just for the return portion. Sometimes called an empty leg on a charter flight. Can be in either direction the passengers just being aboard in only one direction.

I don't understand. What exactly does s/he do?
They do not have to anything on the flight. They are not part of the working flight crew. They could sleep the entire way if they wanted or needed to.

Why are they paid then? BECAUSE THE COMPANY has decided it is worth the cost to pay the employee and transport them.to the place they are needed to do something.

'to assume a duty' They will start doing their "normal" job when they arrive at destination.
This passenger could be a pilot going to replace an sick pilot. Might be going to fetch a new or repaired airplane.

Nothing strange or unusual. YOU are not required to understand.

Deadheading is the practice of carrying, free of charge, a transport company's own staff on a normal passenger trip so that they can be in the right place to begin their duties.

Could be a bus driver on a bus. A train employee on a train. They are travelling to the place they are needed to begin their normal duties. They are being paid for their time to get to the work place.

Some in the past would "volunteer" to assist the working crew. The legal eagles have been over zealous in making it something most will not or can't do anymore. So the employee just sits and enjoys the ride.

Many companies pay for travel time to get an employee from one place to another.

A bus "deadheads" from the garage to the the first pick up point and the start. At the end of day it "deadheads" from last drop off back to garage.

It is a common transportation term and YOU DO NOT HAVE TO KNOW.
You can easily look up the word in this thing called a DICTIONARY.

The employee could also be returning from someplace after completing whatever work they did. It is no concern of the other passengers WHY this person is on the plane and what their required duties are either before the flight or after depending on direction of the"deadhead" travel.

SADLY the entire world world has heard of a few incidents that were not handled well when a last minute deadheading crew member was placed on a flight.

Hundreds of employees are deadheading everyday and have done so for decades. It does become an issue when planes, trains, buses are FULL and paying passengers need to be bumped.

In the immediate moment, nothing, AT THE END DESTINATION, Whatever needs to be done.

A deadhead is a person that follows the Grateful Dead

They are not supposed to work while 'deadheading' because they are not on duty. They have to be fresh for the work that they will do once they've been transported to their destination.

Pilot

The clue is in the phrase "assigned to fly to a particular destination to assume a duty." Pilot Mr Smith happens to be in London. He is needed for duty in New York. So they fly him from London to New York. He is not supposed to work on this flight: he is a a "deadhead". He should wear uniform so that he is recognisably a member of staff, and can sit on a staff seat if necessary.