A host key is the server’s public key in SSH. During connection setup, the client compares the presented key with a known or expected value to confirm it is talking to the intended server, not a lookalike system. The matching private key stays on the server and is used to prove possession during the handshake.
Host keys matter because they anchor SSH identity. If verification is skipped, cached incorrectly, or accepted too loosely, an attacker in a man-in-the-middle position can impersonate the server and intercept sessions. In practice, defenders protect host keys by pinning fingerprints, enforcing strict verification, rotating keys carefully, and warning on unexpected changes. In secure implementations, the algorithm and curve or key type must also match what was negotiated, or validation can fail in subtle ways.