"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.