The term transit, in astrology, is referring to planets in the sky right now and how they are influencing us currently. I use the slower moving planets, Jupiter through Pluto, for analysis in transit readings, because the slower moving transits are slow enough that we can notice their effects on our lives, whereas the transiting moon may only have a noticeable effect on us for 3 hours. Transiting planets affect us by showing us the timing of lessons we may be learning and what we need to focus on during that time in order to make that experience as effective as possible for ourselves. Transiting Jupiter, for instance, is teaching us faith and encouraging us to take more risks in the area of our life it is touching.
The secondary progression technique is based on the common symbolism of a 'day for a year.' This means that a day's worth of literal movement in real life represents a year of movement in secondary progression techniques. For instance, the Sun moves about a degree a day, literally. In a progressed chart, the Sun would have moved about 30 degrees by the time we were 30 years old. This technique is primarily used with the faster moving planets, such as the Sun-Mars (and the angles such as the Ascendant). The Progressed moon, for instance, can tell us where the focus of our heart is during a 2-1/2 year period.