The planets in the sky are in constant motion. Some of these, called fast planets, such as Mercury and Venus, move very quickly. Others, called slow planets, such as Pluto, move very slowly. In any case their positions are never the same.
The transit chart shows the transits that are the aspects formed at a given time between the position of the planets in your natal chart and the position of the transiting planets. Transits are one of the main techniques used by astrologers to make predictions.
There aren't really any good or bad transits. All transits bring something new, new challenges, a question, a greater understanding of themselves and your resources. The transits analysis can help you to determine, at a given time, where you stand in your journey toward personal growth.