So the main part of a computer that controls everything has a specific set of things it can do. Imagine that each of these things can be triggered by a button on a control panel.

Every company making this part of the computer has their own set of things and layout for the buttons to trigger them and you have to pay them money to learn what the things are and how to press the buttons.

RISC-V offers a set of things and a button layout that the community can freely see and use.

In this example a computer program is just the list of instructions saying which buttons to press in what order.