Light clients (or light nodes) are programs intended to be installed on the end user’s machine.
These lightweight nodes connect to the p2p layer and check the block headers, they allow to transform a naive wallet into a septic wallet.
In practice, a user can have a wallet (tikka / gecko) installed on his terminal and configure it to connect to a local lightnode which can be a separate app.
An example of substrate lightnode is Smoldot.