It's short for session. A jam session is when musicians get together and play their instruments and/or sing, improvising new songs.

I'm not sure.... We use sesh as slang. Short for session. Eg. I had a good study sesh this afternoon. I am an English speaker but don't know of a real word sesh. Maybe someone else will be able to teach us both something.

For words like these, your best bet is my looking at Wiktionary or Urban Dictionary. Basically it's short for 'session', perhaps with a goal in mind, such as drinking.