Vanilla Actors with Collision (I.e. CollisionActors) is a mass set of actors that mirrors every applicable vanilla actor in the game, but with added rigid body physics.
This is done because Nintendo baked the entire map into static-compound physics and removed the rigid-bodies from there (vanilla) actors. In other words, we need to re-add the rigid-body physics to have collision in our modded maps. (At least until we can also bake the map, which is on it's way.)
To use C-Actors, edit your map normally with Ice-Spear, but append C to the end of the UnitConfigName to make it a CollisionActor. Then run ActorLoader to download and copy the correct actors into your mod.