A grand trine happens when three astral items A, B and C are trine with each other, creating an equilateral triangle.
Grand trines are like vortexes, the energies flow as if they were following the path of least resistance. They are extremely stable. It's as if the energy was flowing infinitely. It's about perfect equilibrium.
But, grand trines are like windmills: useless if there is no wind. They do not create or activate energies by themselves, like squares do. Something external must activate them, anything.
Those benefiting from grand trines in their natal charts face less challenge to get things started. Everything comes more naturally. They have less pressure to achieve something. Thus, they may get comfortable, and look lazy. It can stir envy from others. It's all about facilities and facilitation. Grand trines tend to trump the other aspects A, B and C are involved in when they are in conflict with it.
When A, B and C are all in fire signs (Aries, Leo, Sagittarius), it is called a grand fire trine. It generates passion, intensity and self-confidence. When they are all in earth signs (Taurus, Virgo, Capricorn), it's a grand earth trine, which generates material stability and common sense. It about being grounded and methodical.
If A, B and C are all in air signs (Gemini, Libra, Aquarius), we are talking about a grand air trine: psychic energies, ideas and abstract thoughts. When they are in water signs (Pisces, Cancer, Scorpio), it's a grand water sign. It's about inner equilibrium, emotions, feelings and empathy.
In synastry, grand trines mixing planets from both partners are a strong support for the stability of the relationship.
Related aspects: trines, sextiles and semi-sextiles
Other minor & rare aspects: grand cross, quincunx & inconjunct, quintile & biquintile, semi-sextile semi-square, t-square and yod