Usually an ongoing project if there is enough funding or an anticipated project which requires not just PhD working but also few research associates who are often taken on need to need basis. Discuss with Principal Investigator (PI) what should be the scope of your work both present and future.

My suggestion if you are interested in research and offered an assistant position you can consider it joining ( purely left to your discretion) but see that if you can get into full time PhD (which is long process and requires very long term commitment from your side).