Moe (萌え, /mo.e/, pronounced "mo-eh" literally "budding", as with a plant) is a Japanese slang word originally referring to fetish or love for characters in video games or anime and manga. For example, meganekko-moe (眼鏡っ娘萌え), or "glasses-girl moe" describes a person who is attracted to fictional characters with glasses. "Moe!" is also used within anime fandom as an interjection referring to a character the speaker considers to be a 'moekko' (moe girl).
The term is subjective and would be a nightmare to untangle if left unrestricted.
You should only use this tag when the image revolves around/mentions the concept of moe or has 'moe' written somewhere on it, not when the image evokes a feeling of moe itself.
If you misuse this tag, the Danbooru Fairy will come and turn you into a wereloli!