A milestone is a significant event in your life. Often a milestone marks the start of a new chapter. For example, the day you graduated from high school was a milestone in your life.
Milestone literally refers to a roadside marker that lists the distance to a particular location. These days, the word is more often used figuratively to refer to significant events in life, like graduating from college or getting married. It acts sort of like the road sign: it's often a moment when you reflect on where you stand in life. A milestone can also be a nonpersonal event that results in a big change, such as a milestone victory or a company's sales milestone.