Sharding is the method by which a bot's code is "split" into multiple instances of itself. When a bot is sharded, each shard handles only a certain percentage of all the guilds the bot is on.
Note: You do not need to worry about sharding until your bot hits around 2,400 guilds. YOU MUST SHARD before you hit 2,500 guilds, however.
There are additional difficulties when sharding a bot that add complexity to your code (one of the reasons you shouldn't shard too early).
Collections do not cache data from all shards, so you can't grab data from a guild in another shard easily.
In order to do anything across shards you need to worry about using
broadcastEval and such (tutorial comming soon!).
Sharded bots often gain very marginal performance increase and might even use more memory due to using more node processes.
If you're using any sort of database or connection, multiple shards may cause issues with multiple processes connecting to a single end point.
/*The following code goes into it's own file, and you run this fileinstead of your main bot file.*/const Discord = require('discord.js');const Manager = new Discord.ShardingManager('./YOUR_BOT_FILE_NAME.js');Manager.spawn(2); // This example will spawn 2 shards (5,000 guilds);