Lock Mouse in Monitor
Hard lock is turned on manually. When turned on, it keeps the mouse pointer within a current monitor unconditionally, until you turn it off.
You can toggle the hard mouse lock by pressing the Lock Mouse in Monitor hotkey.
Soft lock works automatically. When you try to drag the mouse to another monitor, it holds the mouse pointer on a boundary between monitors for a specified amount of time. However, if you continue to drag the mouse longer than this time then it unlocks the mouse pointer and lets it cross the boundary and go to the desired monitor (that's why another name of this lock mode is "tight monitor boundaries").
Such kind of locking prevents unwanted movements of the mouse to another monitor but lets do it without any extra manipulations when it's nevertheless required. You can enable the soft lock on the Mouse panel.