A sextile occurs when two planets sit sixty degrees apart, generally in signs of compatible elements: fire with air, or earth with water. The two functions cooperate easily and support one another without friction.
The distinguishing feature of the sextile is that it is available rather than automatic. Where a trine operates on its own whether or not you notice, a sextile has to be picked up. It describes a talent or an opportunity that will develop considerably if used and will simply sit there if not. Many people carry sextiles their whole lives without ever activating them, and nothing bad happens as a result, which is precisely the problem.
Because the aspect is gentle, it is frequently underrated in favour of the more dramatic squares and oppositions. This is a mistake. Sextiles are where growth is genuinely comfortable, and a life that deliberately develops its sextiles tends to accumulate real capability without the accompanying difficulty.
The elements involved indicate the character of the cooperation. A fire-air sextile tends toward ideas put into action; an earth-water sextile tends toward practical care and durable emotional work.