Discord.js is a node.js module that allows the developer to interact with the Discord API. Use this tag for questions about the usage of discord.js, not for questions regarding the Discord API or Discord in general (for API questions, use the 'discord' tag). Consider also using the 'javascript' tag.
Discord is a proprietary freeware VoIP/chat application designed for communities ranging from gamers to education and businesses.
Node.js is an open-source, cross-platform JavaScript run-time environment for executing JavaScript code server-side.
Discord.js is an object-oriented node.js module used to interact with the Discord API (docs, GitHub).
The official guide of Discord.js can be found here.
Consider also using the javascript and the node.js tag.