Everything is always changing.
The sky, as viewed from Earth, is always changing. At the time that a person is born the sky has a specific configuration which we can later calculate and study -- but, that sky was changing and was different a few minutes later, a few hours later, a few days later. We were born into "change."
Progressions are a symbolic way of looking at that "changing" condition following our birth. In one sense, "progressions" are no different from "transits" except that they have a different time scale. Typically, progressions are based on a "day for a year" time scale rather than being a "day for a day" as transits are.
Progressions are assumed to represent developmental growth of an individual, painting an overall life theme. Since there are many technical approaches to calculating progressions you will need to explore several of the more common methods and evaluate them for yourself. Dave.