The word staple has two main meanings:

1. A staple is a basic, essential item that people use regularly. Rice is a staple food in many Asian countries, meaning people eat it almost every day as a main part of their diet. Bread and potatoes have been staples in European cooking for centuries. The idea extends beyond food: a staple of your wardrobe might be your favorite jeans that go with everything, or a staple of elementary school might be multiplication tables that everyone learns.
2. A staple is also a thin piece of metal bent into a U-shape, used to hold papers together. You push a stapler down, and it drives the staple through the papers, bending the ends on the other side to hold them tightly.

Both meanings share the idea of something fundamental that holds things together or forms a foundation. Just as a metal staple binds papers, a staple food or item is something you rely on and return to again and again.