With frequent locations enabled, your phone is always monitoring your location to see where you go frequently and how long you stay there. After it notices a pattern, after a week or so of the same schedule, it'll start giving you traffic information on how long it would take you to where you normally go at a given time.
For example, when I used it on iOS 7, my phone learned that I go to class in the morning and home in the afternoon and would tell me how long it'd take me to get to each place at certain times of the day. I ended up disabling the feature though because even though it's pretty nifty, it wasn't worth the battery drain to me.