It is a concept in relational databases, where you break a large database into smaller pieces called shards, and distribute them across multiple database servers. Each shard contains a subset of the data.
How it works
A database uses sharding key to know to which instance of a database route the request to.
Example
An e-commerce website with millions of users can shard the users table by region
Shard 1: users from Europe
Shart 2: users from North America
Shart 3: users from Asia
-- Europe shard
SELECT * FROM users WHERE region = 'Europe';
SELECT * FROM users WHERE region = "North America"