Collections are a way of organising your scores into groups, by assigning tags to them. The most common way to use collections is to tag scores that belong together, as part of a bigger project - like a film score, or a concert.
One score could be a part of multiple collections. You can add as many tags as you like to each score, and can even order scores within each collection via drag n' drop. Watch the video below to learn more about how Collections are managed.