How do you "warm a bench" ?

Just a newbie question:

Now if you are seated on some chair or seat, for a considerable period of time, then the surface on which you were seated, turns out to be a little warm.

now what is the science behind this ?

is it possible that the seat gets warmed by the body heat ?

That is exactly the scene. Usually the ambient temperature is less than the body temperature of 37 C. Body heat passes through the clothes to warm the seat.
Incidentally the expression 'Warming the seat' is sarcastically used to denote that someone is not doing anything useful except sitting idle.