Sharding is the partitioning mechanism  of dividing a very large DB into small,faster, manageable parts called Shards  such that all these shards are independent of each other and shares nothing and thus can be distributed across different servers while enjoying all the benefits of horizontal scaling .

Sharding is just another name for “horizontal partitioning” of a database
Horizontal partitioning is a design principle whereby rows of a database table are held separately, rather than splitting by columns.Where each partition consist of some number of rows and forms a part of shard.

With time as the DB grows the time taken to query it increases exponentially sharding helps in scaling the DB horizontally to achieve the performance benefits.