The verb wax is most often found in the company of its opposite, "wane." To wax is to grow larger or increase, whereas wane means to grow smaller or decrease.
As the moon grows towards fullness, it waxes. It wanes, or diminishes in size, as the new moon approaches. This is the most common context for the verb wax, but it is also used to describe other phenomena that grow or increase, particularly those that are cyclical. Figuratively, if you wax eloquent, lyrical or poetic about something, you talk about it at great length and with growing enthusiasm. The noun wax refers to chemical compounds that can be shaped and molded, for example into candles, when warm.