What happens when you jailbreak your iPhone?

I was planning to jailbreak my iphone so that i could download pokemon and stuff. But it said i had to erase my data, when i erase it what happens to my free apps or the ones i have purchased? Would i have to repurchase them or what?

All your apps will stay on iTunes you will not have to purchase/download them again

When it says it'll erase your data, it means the data that's currently on your phone, unless you Synced your iPhone to iTunes prior to jailbreaking you won't have any of your apps, games, calendar information, contacts, etc, unless you synced your phone before you attempted to Jailbreak it