The level of difficulty in dance really depends on where you're taking classes. Each studio has their own scale for each level.
But generally, beginning is for people who have had little to no dance experience. The class will entail basic Jazz dance skills and technique.
Intermediate Jazz will assume dancers have some dance background, and be able to move at a faster pace with increasing difficulty in technique.
Advanced jazz still concentrates on jazz technique, but the teacher will assume the students have multiple years of strong jazz training. The choreography will utilize technique and move at a very fast pace.
Good luck!