Your birth chart is a photograph of the sky at the moment you arrived. It never changes. But the planets themselves keep moving, and as they travel, they pass over the sensitive points in your chart. In Sanskrit this movement is called gochara. In English we simply call it transits.
The fast planets, the Moon and Mercury and Venus, move so quickly that their transits mostly colour days and weeks. The slow ones are where the real story usually is. Saturn spends around two and a half years in each sign. Jupiter spends about a year. Rahu and Ketu, the lunar nodes, take roughly eighteen months.
You have probably heard of Sade Sati, even if you have no other interest in astrology. It is the seven and a half year period during which Saturn passes through the sign before your Moon, the sign of your Moon, and the sign after it. A lot of fear surrounds it, and I understand why. It can be a heavy time.
But in my experience Sade Sati is better understood as a period of growing up. Saturn asks for honesty, responsibility and patience. Things that were built on weak foundations tend to be tested. Things that are genuinely solid tend to get stronger. Many people look back on their Sade Sati as the time they finally became who they were meant to be, even if they would not want to repeat it.
Jupiter's transits, on the other hand, often bring support. Wherever Jupiter travels in your chart, it tends to open doors, bring helpful people and encourage growth. I like to tell people where their Jupiter is heading, because it is good to know where the wind is at your back.
One principle I always keep in mind is that transits do not work alone. They are read together with the dasha that is running. A transit can only really deliver what the dasha and the birth chart already promise. That is why two people experiencing the same Saturn transit can have such different years.
Understanding the present moment is, I think, one of the most practical gifts astrology offers. Not to make you passive, but to help you act with the season instead of against it.