"Slacking off" means doing fun or relaxing things instead of doing the work that you're supposed to do. It sounds lazy.
For example, if a child is watching TV after school, you can say:
I don't think you have time to slack off, Daniel. You have homework to do.
"Slacking off" is bad, but sometimes people admit to it:
A: What did you do this weekend?
B: I just kind of slacked off mostly.