"Trojans" are smaller Asteroids that have been "trapped" at the Lagrange Points L4 and L5 of Jupiter's orbit (explanation below).
Other planets do have such "co-orbiters" as well but Jupiter got by far the most of them, as far as we know now. Interesting to note is, that these millions of asteroids form "families"....
Those points L4 and L5 are about 60 degrees before and after a Planet and explain the planetary Aspect of Sextiles in Astrology.
A Sextile aspect is bringing stability and is "smooth" - yet, one is ahead, just finished the start of a new cycle (orbit) and the other is about to finish a cycle, yet needs to start to discern about change for future or not.... Sextile aspects in general can bring a kind of "laziness" or resistance.
Inner solar system.
White is the Asteroide belt - the green dots are Jupiter's Trojans.
Every planet has a set of five Lagrangian points where much smaller objects, such as asteroids, can maintain somewhat stable positions relative to the Sun and the planet.
Astronomy: Roen Kelly after NASA/WMAP Science Team; Jupiter above: NASA/ESA/A. Simon (GSFC)
L1 to L3 points were discovered by Swiss Mathematician Leonhard Euler a few years prior to Joseph-Louis Lagrange discovered L4 and L5 in 1772 and was the "name-giver".
These 5 points are talking about the "gravitational" resonance of two bodies orbiting. Normally, the two planets/bodies exert an unbalanced gravitational force at a point, altering the orbit of whatever is at that point. At the Lagrange points, the gravitational forces of the two large bodies and the centrifugal force balance each other.
Now imagine a line from L3 to L4 and to L5 - and from L4 to L5 and you get a grand Trine as well as a "pyramid" or better said a Kite.
Astrologically a Grand Trine brings very harmonious energies but often we "overlook" them, as we take the gifts of them as a given and miss to develop or grow them.....
The Kite embodies all the above energies plus opposition as well. As this "picture" does include some focus and challenge through the opposition, there is more possibility or chance to actually "activate" the abilities and gifts hidden in the Grand Trine and overcome the "laziness" of the sextiles to growth.
References and Credits:
Nick A. Fiorenza / Astronomy.com / Wikipedia