What does..
brick.CFrame:inverse()
do?
CFrames are matrices and matrices have inverses.
In the context of a ROBLOX CFrame...
cframe * cframe:inverse() == CFrame.new()
It comes out being useful for some operations with CFrames, although I don't have any good way to explain that.
If you aren't comfortable with using inverse for anything (like me) you can accomplish pretty much everything you would need with toObjectSpace and toWorldSpace.