HP: If you make the average Posts Per Day (1 per day), it stay at 100%. Go under that, it drops in percentage.
MP: The number of days you've made the Posts Per Day threshhold. It goes up a point for each day you've been here.
Exp: Each post you make gives you 1 Exp. Gaining levels gives you more HP.
Mostly, it's just for fun.
...I take it you're from the PADND forums? *smirks* I was from there, then got fed up with certain entities on it and moved here. At any rate, welcome.
And he answered your question, Cap'n. He know Lewd and Lady Wolf. I just suspect he's not used to greeting masterful pirate penguins.