An intermediate certificate, also known as a subordinate certificate, sits between a root certificate and an end-entity certificate in a certificate chain. Certificate chains are used in the context of SSL/TLS (Secure Sockets Layer/Transport Layer Security) to establish secure connections between a client (such as a web browser) and a server (such as a website).

Here’s how it works:

1. Root Certificate: This is the top-level certificate in the chain. Root certificates are issued by trusted Certificate Authorities (CAs) and are pre-installed in browsers and operating systems. They verify the authenticity of intermediate certificates.
2. Intermediate Certificate: These certificates are issued by the root certificate authority. They are used to create a chain of trust between the root certificate and end-entity certificates (e.g., website certificates). Intermediate certificates help to delegate the authority of the root certificate, providing an additional layer of security.
3. End-Entity Certificate: Also known as the server certificate or SSL certificate, this is the certificate that’s presented by the server to the client during an SSL/TLS handshake. It proves the server’s identity and is signed by the intermediate certificate, which, in turn, is signed by the root certificate.

Intermediate certificates are important because they allow Certificate Authorities to keep their root certificates offline in highly secure environments. Intermediate certificates are used to sign server certificates, and if one is compromised or needs to be revoked, it can be replaced without affecting the trust in the root certificate. This flexibility is crucial for maintaining the security and integrity of the SSL/TLS ecosystem.