An erratum is a notice published by a software vendor or Linux distribution about a specific package update. Security errata describe a vulnerability fix, bug-fix errata describe corrections, and enhancement errata describe new features. The plural is errata. Red Hat, for example, publishes security errata as Red Hat Security Advisories.
A security erratum typically identifies the affected product and release, the CVE identifiers it addresses, a severity rating, and the exact package versions that contain the fix. This makes it the authoritative reference for what a vendor has fixed, and in which build.
The erratum matters because distributions often backport security fixes into an older upstream version rather than moving to the newest one. A package whose version number looks old may therefore already include the fix, and comparing it to the upstream project's version number alone can give a wrong answer. Likewise, an erratum being published does not mean it is installed on a given machine.
To act on an erratum, find the one that matches your operating system release and package stream, apply the update with the system package manager, and confirm that the installed package version or changelog matches what the erratum lists. Then restart or reload services that use the updated software, and rebuild and redeploy any container images that bundle the old package, since those do not pick up host updates automatically.