A natal chart describes the sky at your birth. Transits describe where things are now in relation to it — and transit readings are what people book when the question isn't what am I like but what is going on with this period of my life.
The basic idea
The planets keep moving after you're born. Astrologers look at their current positions against your natal chart, and interpret the relationships between the two as describing a period with a particular character.
So a transit reading is time-bound. It's about a stretch of months or years rather than your disposition, which is why people come for one during a transition rather than out of general curiosity.
Why people book them
Almost always a period rather than a question about themselves.
A run of difficulty they want a frame for. A decision they've been unable to make. A sense that something has shifted and they can't say what. Sometimes a stretch where several things changed at once and they want to make sense of the shape of it.
The famous one
The Saturn return is the transit most people have heard of — Saturn taking roughly twenty-nine and a half years to complete an orbit, so it returns to its natal position around ages twenty-nine to thirty, and again around fifty-eight to sixty.
It's become shorthand for the upheaval many people experience in their late twenties. Worth noting the obvious alternative explanation: the late twenties are when a great many people renegotiate careers, relationships and where they live, for reasons that need no astrology at all. The pattern is real; the attribution is the contested part.
What a transit reading can and can't do
It can describe a period in interpretive terms, identify what an astrologer reads as the theme of it, and give you a structure for thinking about a difficult stretch.
It cannot tell you what will happen. Astrology has no demonstrated predictive mechanism, and this is the area where overreach is most common and most harmful — because someone in a hard period is more vulnerable to being told what's coming.
Treat specific predictions as a warning sign, especially specific bad ones. A practitioner who tells you a dangerous period is approaching and offers to help you through it is selling you something.
What people actually report finding useful
Having a difficult period described as a period — as something with a shape and an end rather than a permanent state.
That's not a small thing when you're inside one, and it doesn't require the planets to be causing anything. The frame does the work.
Where it isn't the right tool
If you're dealing with depression, a crisis, or a decision with legal, medical or financial consequences, that belongs with a professional. A reading is reflection, and a responsible astrologer will say so and decline.
When to book one
During or just before a transition, with a question rather than a request for information. What should I be thinking about in this period gets you a useful hour. What's going to happen to me doesn't, and shouldn't.