A version catalog is a list of dependencies, represented as dependency coordinates, that a user can pick from when declaring dependencies in a build script.

The Gradle documentation is here: https://docs.gradle.org/7.3/userguide/platforms.html#sub:version-catalog-declaration

It has a number of nice properties, but the main one is that tools like Dependabot require its use in order to report on Gradle projects.  See here: https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#gradle

POC here: https://github.com/jyemin/mongo-java-driver/commit/cc7d040afece1b58d24b9684deb929be2ed030b7

Note that in Gradle 7.3 it's still in preview, so the feature has to be explicitly enabled.