Emoji (絵文字) is the Japanese term for the picture characters or emoticons used in Japanese electronic messages and webpages (read more about it's 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 it's name in colons e.g. writing
:thumbsup: will display a
You can see all the emoji that GitHub supports above