LFlare is right. You have a pregnant female guppy there. It's a fuzzy pic, but it looks like she's got a while before she pops. What you need to look into now are live birthing chambers to keep her from eating the babies and deciding whether you'll let them into the main tank to take their chances or put them into a separate baby tank for a few months. I've done both. You have more control and loose less with the separate tank, but you can end up with stronger fish the other way. Guppies will cannibalize the babies.
Yes definitely pregnant. As Dalton says, you may want a birthing chamber to put her into.
I didn't learn this process quick enough when I was a teenager and went from 1 tank with 5 guppies in it, to 5 tasks with 600 guppies in less than 6 months! My parents were less than amused...
Good luck with your new hoard of little breeders.