The Trine is a geometric aspect between two planets that are about 120 degrees apart (give or take an orb of a few degrees). It shows there is an easy-flowing relationship between the planetary energies, and often indicates natural talent and ability specific to the planets, signs and houses involved.
Planets in trine aspect are usually four signs apart and typically are in signs of the same element (Fire, Earth, Air or Water). If one planet is at the end of a sign and the other planet is at the beginning of the 5th sign beyond it, and the trine aspect orb encompasses both planets, this is called a dissociate aspect or out-of-sign aspect. In this case, the planets are in signs of a different element. For example, a planet in Aries (Fire) would normally be trine to a planet in Leo (also Fire) if the degrees are within orb. However, if the Aries planet is at the end of the sign, it might be within orb to trine a planet in early Virgo (Earth), if the trine orb is wide enough. In this case, some astrologers feel this weakens or even invalidates the effects of the trine.
The trine aspect is usually interpreted as a positive influence, since the planetary energies combine, cooperate and work together easily. However, sometimes it could also be too much of a good thing. Trines can also be easy to take for granted, so they may also signify latent talents and untapped natural abilities. There is no such thing as an "all good" or "all bad" aspect or planet.