lukewarm, warm, tepid, mild
溫 centers on moderate heat that is comfortable rather than intense: lukewarm, tepid, or mild warmth. The meaning extends naturally to gentle heating or warming something up.
溫 is the traditional form of 温. It combines 氵 (water) with a phonetic component historically written as 囚 over 皿, which contributed the sound and the idea of warmth. The modern Japanese simplified form 温 replaces the upper right part with 日.
Water 氵 is being gently heated in a covered dish (囚 over 皿), making it lukewarm and mild.
For オン, imagine turning on a warm bath: on -> オン, and the water 氵 becomes comfortably warm.