Constraints are basic physical actors in Unreal Engine 4. Imagine that physical rules need something to be presented with, such as a tool, switch, calculator, or container. It's called physic actor, and physic actors that are responsible for simulating the physical behavior between two objects are known as physics constraint actor.
It works in a similar way to a minicomputer. You can connect two inputs to it, and it calculates how these two should play with the physical rules in the game. Finally, it applies the changes to the object as real-time processing during the game play.