Shadow Mend is a healing spell. It heals you for a big chunk of health but then damages you over time afterwards. Useful if you need it to stay alive or in solo content but it’s not really something you’re supposed to be spamming.
Dispel is what it sounds like. Take a negative effect on someone and get rid of it. Again you won’t really be spamming this as people simply shouldn’t need dispelling that frequently.
The main resource of Shadow Priests is insanity, not mana.