Interval runs are runs in which you have stretches of high intensity runs either for time or distance followed by a recovery period for several rounds. Usually the high intensity part of the interval is double or more of distance or time in comparison to the rest. Examples would be:
400m as hard as you can
5 minutes run at threshold pace
2 mins rest
Which kind of intervals are best suited for you depends on which distance you are preparing for in your races. Intervals are a great tool to improve your speed, however most likely it is a part of your training you might enjoy the least, especially when you just starting out.