To prevent scalability issues, Zilliqa uses a method called sharding.
Sharding is a type of database partitioning — it breaks down databases into smaller, faster, and more easily manageable parts called data shards. These data shards are thus fractions of a database, meaning that the data shards only contain fractions of the information.
Therefore the shards are meaningless in themselves; their content is only valuable when put together. Sharding also allows for all data shards to be stored across multiple servers.
In the case of Zilliqa, sharding is used to divide all the work of verifying transactions and securing the network across the network. The verification of transactions is done by a group consisting of a maximum 600 nodes, called a shard. Whenever the threshold of 600 nodes is reached, a new shard is created.