The Rigidbody component adds collision to a GameObject

Rigidbodies are components that allow a GameObject to react to real-time physics.

Explanation:

Rigidbodies are components that allow a GameObject to react to real-time physics. This includes reactions to forces and gravity, mass, drag and momentum. You can attach a Rigidbody to your GameObject by simply clicking on Add Component and typing in Rigidbody2D in the search field.A rigidbody is a property, which, when added to any object, allows it to interact with a lot of fundamental physics behaviour, like forces and acceleration. You use rigidbodies on anything that you want to have mass in your game.You can indeed have a collider with no rigidbody. If there's no rigidbody then Unity assumes the object is static, non-moving. If you had a game with only two objects in it, and both move kinematically, in theory you would only need a rigidbody on one of them, even though they both move.