In simple words
An aspect is an angle between two planets. A trine means they are about 120° apart.
It is a flowing angle: things come easily between those two parts of you.
In a birth chart
Trines show natural talents and areas of life that feel easy. The only risk is taking them for granted; a gift used on purpose becomes a strength.
Vesper counts a trine when two planets are within 6° of the exact 120° angle in a birth chart, and within a tighter range for day-to-day transits.
As a transit to your Sun
Sun trine your Sun. The Sun forms a flowing angle to your natal Sun. Your energy and identity are well supported for a few days.
Moon trine your Sun. The Moon forms a flowing angle to your Sun. Feelings and purpose cooperate, so what you set out to do feels natural.
Mercury trine your Sun. Mercury forms a flowing angle to your natal Sun, so communication and self-expression come easily.
Venus trine your Sun. Venus forms a flowing angle to your natal Sun. Affection and pleasure come easily.
Mars trine your Sun. Mars forms a flowing angle to your natal Sun. Energy and purpose work hand in hand.