Session resumption is a TLS feature that lets a client and server reuse state from a previous connection instead of performing a full handshake again. Depending on the TLS version and implementation, this may use a session ID, a ticket, or other cached session data to shorten connection setup and reduce CPU cost.
It matters because the resumed connection inherits trust decisions from the original one. If session state is handled incorrectly, an attacker may be able to bypass expected checks, confuse identity bindings, or keep using stale security context after policy changes. Defenders care about where session caches live, how long tickets remain valid, and whether resumption is allowed across certificate changes, load balancers, or protocol upgrades. In real systems, resumption improves performance for web, embedded, and long-lived services, but it also creates another stateful path that must be validated carefully.