In astrology, a progression is simply how the planets move around your birth chart. Your natal chart indicates your character and potential, while your progressed chart is calculated by your astrologer using a centuries-old method.
Your natal chart will always be the same, but your progressed chart will change every year of your life. Progressed charts help understand changes in your life and your personality and get a hint as to what may be happening in that particular year.
Progressed charts are not to be confused with a solar return chart, which is calculated for the anniversary of your birth and uses a different procedure.