Stuff is a technical term used to describe the qualities of a pitch that make it hard for the batter to make contact. For fastballs, stuff is generally measured in raw speed, since the amount of break is limited. For other pitches, the important characteristic is the amount of movement or break.
Having good stuff is important for any pitcher, but no more so than control. A pitcher with good stuff but poor control may record many strikeouts, but will also tend to surrender a large number of walks and wild pitches. The history of baseball is littered with pitchers like Steve Dalkowski who had great stuff but were ineffective because they couldn't throw it consistently for strikes. If the amount of break on his pitches varies, a pitcher can even suffer from having too much stuff because it makes them very hard to control or to catch.