To moisten is to make something a little wet, or to become a little wet. You moisten a sponge, moisten the soil, moisten a stamp, moisten your lips. Eyes can moisten when someone is about to cry — a little water, not a flood. You do not moisten a thing by pouring a bucket on it. That is soak or drench. Moisten stays on the gentle side of wet. The ending -en turns the adjective moist into a verb, the way sharp becomes sharpen and wide becomes widen. Though actually, English only does this with a small set of adjectives. You cannot add -en to every word and hope.