Constraints allow you to control an object beyond its standard properties (e.g. translation, rotation, scale). Some constraints can set limits on these properties (and their hierarchical relationships), while others allow you to copy properties from one object to another.
Examples of where you'd use constraints:
Make a character's eyes follow a target. Because you can animate the strength of a constraint, you can easily turn this on or off (or blend the amount). You can also set limits on this movement. In this case to prevent the character's eyes from moving off the face.
Make a character's legs automatically bend at the knees. Easily position a chain of bones while ensuring the joints only bend in one direction.
Make all the wheels on a vehicle rotate together. Instead of animating each wheel individually, you can make all the wheels copy the rotation of one target wheel. Then make that target wheel rotate based on the distance it (or a target) has moved. All the wheels will now rotate just by moving your vehicle, no need to animate any wheel rotations.
Make the hands on a clock rotate. Animate only one object to make both hands move proportionally.
Move the facial features on a character. Use just one target to move the eyes, nose, and mouth of a character. Because you can set a custom strength on this, you can make some objects move more. For example, you can make the nose move slightly more than the mouth and eyes, creating parallax and the illusion that the face has depth. Similarly, you can make the pupils move more than the white of the eyes.
Copy translation, rotation, or scale from another object. You can set the strength and a limit on this. This becomes particularly powerful when you animate these properties. Note that strength can also be negative.
Select an object you want to apply a constraint to. This object is the owner of the constraint. In the Inspector, click on the plus icon next to the Constraints header. A menu displays the constraints that are available for the type of object you've selected (for example, the IK constraint is only available if you have a bone selected).
Note that you can add multiple constraints to this object. This list is sortable because the order in which these constraints resolve is important. Click and drag to sort this list.
Constraints appear in the Hierarchy, below their owner. To view or modify a constraint's properties, select it either in the Hierarchy or by clicking on it in the Inspector.