Custom Emoji

Custom Emojis are a feature that allows you to use any image provided by your instance as an emoji. You can use them in places like notes, reactions, chat, your profile, your name, etc. To use a custom emoji in one of those places, press the emoji picker button (if available) or type : to display emoji suggestions. If a string of the form :foo: is found in the text, the foo part will be interpreted as a custom emoji name and it will be replaced by the corresponding custom emoji when displayed.

You can see which custom emojis are available on your instance by visiting /emojis on your instance's Misskey site.