In the real world, we are surrounded by constraints. For example, gravity constrains us to the ground, dogs may be constrained by the length of their leashes. To simulate these limits in the 3D animation world, these relationships between objects are established by the animator using constraints.
Constraints are tools used to create relationships between objects. You can use constraints to make a character pick up an object, have a camera follow a character, manipulate objects in relation to each other or use the keyboard or mouse to trigger events.