If a computer were a boardgame, a program would be the instructions for the game. The program tells the computer how to start, what to do in certain particular situations, and when to stop.
It is not a perfect analogy, of course, but it will do fine to get us started.
One way a program is like a boardgame is that every player must take turns.
Sometimes, when a complex computer program is running, it can seem as if there is a whole world inside there, all moving and changing simultaneously, but it is only an illusion.
The computer can only process one instruction at a time.
However, it can process the instructions so fast that to us it seems smooth and almost alive.