According to that infallible information source Wikipedia, a milestone (from the Latin milliarium) is one of a series of numbered markers placed along a road or boundary. Milestones are constructed to provide reference points along the road. This can be used to reassure travelers that the proper path is being followed, and to indicate either distance traveled or the remaining distance to a destination.
We should take time to pay attention to milestones in our life. Talk about them. Celebrate them. Weep over them. They can help us to be sure we know where we are in life. They can also help us to remember where we’re going.