It's a little hard to grok, so no sweat. Let me try an example:
It's 11 in the morning and your mother is kindly asking you from two rooms over "Felkelsz?" - Are you getting up? She wants you risen and ready, so she uses the finitive form with the prefix. Your answer, naturally, is "Kelek." - I'm getting up. You're in the process, but it's uncertain yet if you'll manage to get out of bed. Or you don't really plan to.
It might make a bit more sense once we get to talking about the past:
- "Felkeltem." - I got up, out of bed, I'm ready for the day.
- "Keltem." - I was in the process of getting up. Maybe I succeeded, but maybe the pillow suddenly started looking very snuggly and I lay down again.