Understanding the very basics of Unity Physics

For any kind of interaction with other GameObjects, we will need at least some basic physics.

Colliders
You always need a Collider, if you want your GameObject to interact with something. Unity provides various Colliders pre-built into it.

Example on a Sphere GameObject

Rigidbody
To keep this simple, we stay at demonstrations:

Neither a Collider nor a Rigidbody added

Rigidbody added on the white Cube

Rigidbody added on both cubes