So what exactly does foam rolling do? Well, when you workout, you are demanding a lot of your body. The pushing, pulling, straining, and sweating are all done with the end goal of keeping you healthy and looking your best. At the very basic level of anatomy, when you exercise effectively you create micro-tears in the muscle fibers. Don't be alarmed though, this is a good thing! People don't like to associate a healthy workout with ripping their muscles apart, but it's not as dramatic as that; these tears are very small, only at the individual fiber level.
When you recover after a good workout, your body lays down collagen fibers in the muscle to start the healing process. These fibers, in a perfect world, would line up with the line of pull in the muscle, and everything would be nice. However, it doesn't always work like that. What actually happens looks more like a game of Pick-Up Stix (probably aging myself saying that, but oh well...) Fibers laying every which way create what is called an adhesion, or a sticking point in the muscle. To people like you, this can simply feel like a knot that you want to massage out. Other times it can simply be a soreness, with no point-specific tenderness.
So here's where the roam roller comes into play. Using the roller, press along the entire length of the targeted muscle for 10-15 repetitions. It's not a race though - take the time to move through the muscle body slowly. A good visual to use is to picture your muscle as play-doh or clay; you wouldn't want to just start ripping that stuff apart when you first start using it! You want to ease into it, taking the time to make it more pliable and it will lengthen out. One last tip - if you find a specific point that is more sore than the other areas, hang out on it for an additional 10 seconds, and try to get that adhesion to release and loosen up.
Now that you know, make sure to incorporate foam rolling in your pre-workout routine. Good luck!