What a transit is
Today's sky passing over your fixed chart.
Your birth chart froze at the moment you were born and never changes. The sky keeps turning. A transit is the angle today's planets make to points in your birth chart.
An example
For example: Saturn is at twelve degrees of Pisces today. If your Moon is at twelve degrees of Gemini, Saturn is squaring your Moon — a stretch where things feel emotionally tight.
What does a transit compare?
The chart is fixed, the sky is moving.