Transit : When a planet crosses from one sign/bhava to another sign/bhava it is considered as a transit of the planet. In Vedic astrology, the date of transit is different than the date of transit considered in Western system due to ayanamsa.
Ayanamsa : Ayanamsa is the correction of 50 1/4 seconds of the equinox - a point where the ecliptic intersects the celestal equator. The first degree of Aries in the hindu system of astrology differs presently by 21/22 days from the western system.
After readings for sub-periods you have the effects of transits of the three planets - Saturn, Jupiter and Rahu (North Node). They can also modify the results of dasa. So, while reading your horoscope you have to look at the dasa periods and the transits together.