A trine between two planets in an astrological chart occurs when the planets are one hundred twenty degrees apart. Usually the two planets will be in the same element, giving them a solid common ground.
A trine is a soft aspect, meaning its effects in your life are positive, even so easy as to seem effortless. Trines bring sweet benefits to your life: effortless accomplishments, opportunities that fall right into your lap.
Since this aspect brings such good things your way with no special effort on your part, it can make you lazy as a result. After all, if you don't have to work for your gifts in life, you could easily start thinking you deserve all the wonderful things life has to offer, and that you shouldn't have to work for them or pay your dues in any way! That is the inherent lesson of a trine -- to stay humble even when you're living on easy street.
For example, a trine between the Sun and Mars in your chart would result in a strong sense of yourself and your goals in life. Whatever you decided to pursue -- a love affair, a promotion at work, a winning game for your team -- would seem to fall into your lap with no special effort on your part.
Competition would melt away, leaving you to enjoy your prize. Sounds like a charmed life, right? The trine does bring you those benefits, but urges you not to become complacent. Enjoy your gifts in life, but remember to use them to make the world a better place, not just your own life.
Here's a chartwheel example of a trine: