I had it explained to me that it's the number of fifths that the babies head is below the pelvic brim... So two is just under half, and from three (just over half) baby's head is considered engaged. But full engagement is 5/5... ie, all of the head is below the pelvic brim.
If you are 2/5 now you're doing well - I'm over 40 weeks and my little boy is only 2/5 since a few days ago!!