Mates are the most important tool within Assemblies
Menu Bar→Insert→Mate OR
CTRL+LMB Both Entities you'd like to mate, LMB Pop-Up Menu Icon
NOTE: If you have the items pre-selected when you enter the Mate Command, SolidWorks will attempt to automatically select the Mate Type; this can be useful and/or detrimental
Use Coincident Mate to connect multiple Entities
Concentric is a commonly used Mate that can be used to make multiple things centered with each other such as two Arcs, an Arc and a Line or even an Arc and a Point.
Make to faces Parallel (you could also try Perpendicular, but this is used less frequently)
Tangent is used to mate Cylindrical Faces to Planar Faces
Allows you to enter a distance between your two selected entities
NOTE: You can change the Alignment AND the ± of the Value
Allows you to enter an angle between two planes
NOTE: You can change the Alignment AND the ± of the Value
Some mates can be reversed while still being enabled. Think of a Concentric Mate between two Coins; Flipping the Alignment would change a coin from Heads-Up to Tails-Up etc.
Sometimes a Coincident Mate must be Flipped; A Face of a Coin touching the ground (face down) or the Face of a Coin touching the ground (face up (not possible in reality, but SolidWorks has this))
Once Mates have been inserted, the Mates Folder appears at the bottom of your Design Tree. You can expand the folder to show all the Mates in the Assembly. These are listed in the order you created them in and may be edited from here.
It is more likely that to Edit a Mate, you'll want to select the Component and then LMB the Properties Tab which will give you a filtered list of all the Mates attached to that selected Component
RMB Mate→Edit Feature
NOTE: Generally you cannot change a Mate Type. If this is your goal, it's likely easier to Delete (RMB→Delete) it and recreate a new Mate
If you want to make many identical (type) mates to a single reference, you may use a Multi-Mate
This often works best when there are many things that are coincident or parallel to a single source such as screws in a plate
When in Mate Command→LMB Multi Mate Icon→LMB the item you want as your Mate Parent (in the top box)→LMB subsequent items to mate