A program is a set of instructions that tells a computer what to do. For example, a very simple program can tell a computer to add two numbers together or display a message on the screen. The browser you are using to read this is also a program, but a more complex one.
Very Simple Definition:
A program is like a recipe that tells a computer what to do. It gives the computer a list of things to follow. For example, a program can tell the computer to show a picture or play a song.