A Station is when a planet changes direction from direct to retrograde, or from retrograde to direct. Astrologically, a planet's energy is extremely intensified for at least a day, often up to a week, when it becomes stationary. The planet's energy is powerful and dominant.
Think of a pendulum as it swings. When it reaches one side, it slows down and pauses (becomes stationary) and then starts to go the other way. In the same way, a planetary station is when a planet slows down and seems to hover around the same degree, and then resumes motion in the opposite direction.
When a planet pauses and changes direction like this, it is called a Planetary Station. We say "Mercury is making a station" or "Venus is stationary."
When someone is born within a few days of a planetary station, that planet becomes very strong in their chart, often seeming to "take over the chart." When a planet stations by progression, it can be a significant turning point in the person's life, particularly relating to where the planet is located in the natal or progressed chart, the house(s) it rules, and so on.