to be way behind is a phrase meaning you are not completing something on time, maybe you have a deadline and a paper is due on Monday and on Sunday you have not started the paper yet. Then you would be way behind in your work. If you were running a race and everyone in the race had finished but you were still running you would be way behind. It can also mean to struggle with something like a school subject. for example I am way behind the rest of the students in the class they all seem to understand the material and I am very confused. the way in way behind is used for emphasis to suggest that you are very behind and not just a little behind.

Way (intensifier) = super, terribly, so Behind = late or too slow especially when you have to do something. Achieve less progress.

thanks