The Rigid Body Component adds physics properties, such as mass and velocity, to the entity. The component will simulate physics for the component and set the position and orientation of the entity accordingly.
Adding a collider - needed for collision
If you add a Collider Component to the entity then the collider will be used for rigid body collision, with the center of mass being at the same location as the Rigid body entity.
Adding many child colliders - for advanced collision shapes
Adding a single Collider Component and a Rigid Body Component on the same entity is a bit limited - the collider will always be centered in the entity and you cannot move it. The solution is to put it on an entity below the rigid body component in the hierarchy. This way you can place the collider wherever you want (relative to the center of mass) and how many entities you want.
So if you’d like to make a chair consisting out of 6 colliders (4 legs + back rest + seat), you first create a root entity with a Rigid Body Component. Then you create 6 entities with box collider components and put them as children of the root entity. Scale and position them correctly, and you’re done.
Kinematic vs Dynamic
By default, the Kinematic checkbox is unchecked. This means that the Rigid Body is Dynamic. A dynamic rigid body is affected by external forces, a kinematic is not.
If you set the body to be dynamic, it will fall according to gravity. If you set it to Kinematic, it will not. Same with collisions: a Kinematic body cannot be pushed by other bodies.
You can set the Initial Velocity and Initial Angular Velocity in the panel. This defines what velocity the body should have when you press play.
Drag is a value between 0 and 1 that defines how much “air resistance” the body has. If Linear Drag or Angular drag is zero, then there will be no air friction. 1 is the maximum value and the body will barely move.