General Things an Associate Producer Does
The associate producer is still one of the film jobs in the producers department, so it usually works on the overall completion of the film rather than one specific technical element.
The producer oversees the production of the film itself, approving budgets, organizing production, and everything else in this area in a high level fashion. An associate producer is not going to have this kind of power generally, and will often instead work at one level of this organ making sure certain things get done.
An associate producer may only work in the office of the production company overseeing the production and may end up doing several producer functions on several different projects at once. They may also just work on a certain part of the project, such as when shooting on one location during a narrative production or on a certain segment of a documentary film. Perhaps they oversaw the script development, post-production, distribution deals, or some other subset where the producer's department had to be there but the lead producer could not be there. They may even just serve the function of an assistant producer, providing support to the main producers in their jobs.
Whatever the determined role is, the associate producer will not be a full producer and will likely not be a participant in the production, meaning that they will not receive above the line profit sharing. If it is a union governed shoot they may or may not have to be part of the Producer's Guild, but usually an associate producer will not, and will be hired to only do tasks that other union positions do not have to do. Oftentimes, a co-producer will do the same functions of an associate producer, but they are still above an assistant producer on most sets.