In simple words
An aspect is an angle between two planets. A sextile means they are about 60° apart.
It is a flowing angle: things come easily between those two parts of you.
In a birth chart
The planets help each other easily, but the gift usually needs a little effort to use. Sextiles show skills that grow quickly with practice.
Vesper counts a sextile when two planets are within 4° of the exact 60° angle in a birth chart, and within a tighter range for day-to-day transits.
As a transit to your Sun
Sun sextile your Sun. The Sun forms a friendly angle to your natal Sun. For a few days, your vitality and purpose get gentle support.
Moon sextile your Sun. The Moon forms a friendly angle to your Sun. Your feelings support your goals, so small efforts go further than usual.
Mercury sextile your Sun. Mercury forms a friendly angle to your natal Sun, supporting clear thinking and helpful exchanges.
Venus sextile your Sun. Venus forms a friendly angle to your natal Sun, bringing ease to relationships and pleasures.
Mars sextile your Sun. Mars forms a friendly angle to your natal Sun, giving extra energy that is easy to use well.