A shard in JaxNet is a shard-blockchain together with the group of nodes that maintains it. Shards together with the beacon chain constitute JaxNet network.
Currently, in the majority of blockchain protocols each full node stores the entire state (account balances, contract code and storage, etc.) and processes all transactions. It provides a large amount of security, however, it greatly limits the transaction throughput of blockchain networks. Sharding is considered to be the most promising approach to overcome blockchain scalability issues.