Emoji (็ตตๆๅญ) is the Japanese term for the picture characters or emoticons used in Japanese electronic messages and webpages (read more about its history). You can use emoji in GitHub issues, markdown files, pull requests, commit messages... basically almost anywhere you see a textfield.
To include an emoji inline simply wrap its name in colons e.g. writing :thumbsup: will display a
You can see all the emoji that GitHub supports above
Have fun!
See also: the Emoji Searcher and the Emoji Cheat Sheet