Mass Effect 3 - Side Missions

Side Missions are any missions which don't have either a "Priority" or "N7" designation. They are not required to complete the main story but completing them will get you upgrades, gear, War Assets and experience which will both help you finish the main missions and help you get the best endings.

It's very important to do Side Missions as soon as they are available because some are only available up to a certain point during the main story, after which they will disappear.