"at once" means "immediately, right now, instantly"
"on time" means arriving or doing something at the correct time, keeping to a schedule, keeping your agreement of when to do something, not being late.
"at last" means "finally". "It has been a long day, so I am glad to be home at last!"
"in advance" means "before something is finished/required". So if I pay for something in advance, it means that I pay first, and then get the finished product afterward (which requires some trust that I will still get the item I paid for even if I pay you early). If I "get an advance" it means that I am paid some of my salary early, before the job is finished. If I read extra chapters for my class before they are required in class, then I am reading ahead or studying those chapters "in advance".
@chiqip I can understand that. I just didn't want to answer questions that I find easy, and not truly help the person to learn. So I thought I'd ask so that I could understand what you needed. To be honest, I sometimes overthink things too.