What is self and how can I use this thing called self?

On the devforum, I see that programs typically use something like this:

function CameraModule:LockMouse()
    if self.activeCameraController then
        local mouseLockOffset = self.activeMouseLockController:GetMouseLockOffset()
        self.activeCameraController:SetIsMouseLocked(true)
        self.activeCameraController:SetMouseLockOffset(mouseLockOffset*3)
    end
 end
--This is just an example of someone using the 'self' thing.

If you see this, there is a thing called self. What is self, and what are the practical uses of using self in functions like this? Any help is appreciated!