Mesh colliders are, generally speaking, a bad idea in Unity. The reason being that they cause collision based on the detailed mesh that forms the object. This is bad, basically because this generates many collision points. Consider this object:
As you can see, it’s a basic cylinder (a cup), so a box collider would add eight collision points. A mesh collider generates a point for each of the 496 vertices.
So, whilst it’s more accurate, it uses more resources.
Okay, so now I’ve said why you shouldn’t use a mesh collider, I’ll cover how to use them.
Firstly, you need to add a mesh collider:
There are three important things to note here:
There is a mesh collider
It is convex
1. The convex flag allows the collider to collide with other colliders
It is not a trigger
1. The trigger flag turns the collision into a programmatic notification only
The manual for this is here.