Transits describe the relationship between a planet's current position and its position at the moment you were born (your birth chart). The planets have kept moving since you were born.
When a planet's position today makes an aspect to a planet in your birth chart, we call it a transit.
The Sun, for instance, goes around the zodiac once a year and makes a major transit every three months (each quarter of its 12-month cycle). Every planet has its own cycle of a different length: ☽︎ Moon ~28 days, ☉︎ Sun 1 year, ♃︎ Jupiter ~12 years, ♄︎ Saturn ~29 years.
- ♄︎ Saturn ☌︎ natal ♄︎ Saturn (the Saturn return, around age 29): reflecting on discipline, responsibility, the passage into adulthood.
- ♃︎ Jupiter ☌︎ natal ☉︎ Sun (once every ~12 years): expansion, luck, optimism.
- ☽︎ Moon ☍︎ natal ☉︎ Sun (every month): a small tension between feelings and identity, a full-moon feeling.
Orb
Two planets make an aspect exactly for only an instant. The orb is the buffer zone that decides what still counts as the aspect. A solar return, say, starts 3° before the exact angle and ends 3° after. Different astrologers use orbs between 1° and 10° for different purposes.