A light client is a Bitcoin application, such as a wallet, that does not store the blockchain. Instead, this application simply queries other nodes for the specific transactions and blocks in which it is interested.
Light clients are used by wallets to inform users of their balances and transactions, and they are less private and trustworthy than nodes but are more convenient and less demanding of disk space. Most mobile wallets and some desktop wallets are light clients, because running an entire node is not yet efficient on all types of devices. While light clients usually connect to a central server, most light clients have the ability to connect to any Bitcoin node in order to preserve privacy and trustlessness.