What does it mean when it says "pre-order" on iTunes when you're trying to buy an album or song?

I want to buy a song buy Natasha Bedingfield (Pocketful of Sunshine) but when I open up iTunes to try and buy it, it says pre-order. What does this mean and what happens if I click it? Is it possilbe to get the song without buying the album in a store? Please help!