Mates create geometric relationships between assembly components. As you add mates, you define the allowable directions of linear or rotational motion of the components. You can move a component within its degrees of freedom, visualizing the assembly's behavior.
Some examples include:
A coincident mate forces two planar faces to become coplanar. The faces can move along one another, but cannot be pulled apart.
A concentric mate forces two cylindrical faces to become concentric. The faces can move along the common axis, but cannot be moved away from this axis.
Mates are solved together as a system. The order in which you add mates does not matter; all mates are solved at the same time. You can suppress mates just as you can suppress features.
Other topics about mates include: