You can configure custom emojis for your channel. These emojis will be available in the emoji picker. Users can also use them by with their short name (for example by writing ":shortname:").
You can use emojis in the chat using ":shortname:". The short name can start and/or end by a colon (:), and only contain alphanumerical characters, underscores and hyphens. It is strongly recommended to start them by a colon, so that users can use autocompletion (by typing ":" then press TAB).