From Diablo Wiki
Main in video game discussion generally refers to a "main character," one that a player spends most of their time on. This is in opposition to an Alt, an alternate character that doesn't get as much love in time or quality of gear.
Many RPGs such as WoW require tremendous amounts of time to maintain one character, and thus Alts are usually that; played much less often. ARPGs like the Diablo games generally do not require complete devotion to one character, and thus most devoted players operate multiple characters, rather than just one.
This is less true in Diablo 3 than in earlier games in the series, since
Main may refer to other game features than characters, of course. Mainstat is the common term for Str, Dex, or Int, depending on which of those is the damage-boosting modifier for a character class.
- See more Reference terms.