Milestones can be unlocked after doing something, be it a certain task single or multiple times or something entirely unexpected. You can check your milestones by going to the others menu, located at the top part of the client, as the image on the right suggests.
Some milestones may also grant you a bonus, such as extra HP and/or MP%, exp%, %elite chance and damage.
You can check whether a milestone grants you a bonus or not by expanding the desired category in which your milestone fits. The bonus should appear alongside its description, like shown in the image.