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).
You can create a bot at Discord Developer Portal.
The official guide of Discord.js can be found here.
The official documentation for Discord.js can be found here.
Consider also using discord, javascript and node.js tags.