An intermediate CA key is a private signing key used by a subordinate certificate authority (CA). The CA uses it to sign certificates, including certificates for other subordinate CAs or for servers. In a typical TLS certificate chain, the server certificate links through one or more intermediate CAs to a trusted root CA.
Intermediate CA keys matter because clients use certificate-chain verification to decide whether to trust a server. If an intermediate is distrusted, compromised, expired, or otherwise invalid, a chain that depends on it may fail verification and the TLS connection may be rejected. Defenders can investigate such failures by inspecting the full chain and checking which intermediates it contains. CA operators protect these keys carefully, since misuse can undermine trust in certificates issued beneath them.