I presume you know what SJ is so I'll just explain the jumpcross part!!
Jumpcross is effectively showjumps (green/yellow polyjump style) over XC terrain. They can include going through water, up/down slopes (poss steps/ditches) but with the reassurance of knock down fences rather than solid.
A nice introduction to XC riding if you or your horse haven't done it before and they tend to have training/competition days quite regularly through the summer.