A transit is the position of a moving planet at a given moment as it interacts with the fixed positions in your natal chart. Transits show the timing and quality of events as the sky's current configuration activates parts of your birth chart.
Short answer
A transit is the position of a moving planet at a given moment as it interacts with the fixed positions in your natal chart. Transits show the timing and quality of events as the sky's current configuration activates parts of your birth chart.
In depth
Your natal chart never changes — it is a permanent record of the sky at your birth. The actual sky, however, keeps moving. When a moving (transiting) planet forms a meaningful angle to a planet in your natal chart — a conjunction, opposition, square, trine, or sextile — that aspect is a transit.
Different planets produce transits at different speeds and timescales. The Moon transits each natal point monthly (subtle daily moods). The Sun transits each natal point annually. Personal planets (Mercury, Venus, Mars) bring weekly to monthly themes. Outer planets (Jupiter, Saturn, Uranus, Neptune, Pluto) produce multi-year transits that mark major life chapters.
Saturn returns (Saturn transiting back to its natal position, ~age 28-30 and ~age 58-60) and Jupiter returns (~every 12 years) are among the most famous and watched transits because they consistently mark life-stage transitions.