ITunes. Transfer Purchases? What Will Happen?

So I had an itunes account on our regular computer , then i got a laptop for christmas and put itunes on there. When I plugged my ipod into my old computer after all my new songs from my laptop itunes were on my ipod, it popped up erase and sync or transfer purchases. what does transfer purchases do? Will it put the songs from the old computer onto my ipod as well as my newer songs? I'm so confused and I bet after reading this you probably are too. Sorry , but just bare with me. Haha (: