Mainly means mostly or for the most part, but not completely. When something happens mainly on weekends, it happens on weekends more than other days, though it might occasionally happen on a weekday too. When a library is used mainly by students, students make up most of the visitors, even though some adults and younger children visit as well.

The word helps you describe situations where one thing dominates without being absolute. A soccer player might be mainly a defender but sometimes play midfield. A book might be mainly funny with some serious moments mixed in. Your allowance might come mainly from weekly chores, with a little extra from special jobs.

Mainly is useful because life rarely divides into perfect categories. It lets you be accurate without exaggerating. If you say the test was mainly about fractions, you're being more honest than saying it was only about fractions, especially if a few geometry questions appeared too. The word acknowledges that while one thing predominates, other elements exist around the edges.