all DVCSs are ultimately the same thing, they are all just providing a human interface to the same data structure. i am not saying this dismissively - human interfacing is an extremely difficult problem where a lot of breakthroughs can be made (and some of these breakthroughs will involve interesting algorithms, see pijul).

so to answer your question, jj allows you to do the same set of things you can do with git, but with (arguably) much better UI/UX.