The url you posted leads to a generator for a maven / gradle project. It essentially gives you a hello world quarkus project, including Dockerfiles, mavens pom.xml, etc. You can quickly select often used dependencies / libraries you want to have included. They are put in the maven / gradle dependency lists of the project you are creating. That makes it faster to start a new application.
Are you confused by the wording extension here? They use it synonymously with dependency / library, in this case, maven/gradle dependencies. They are probably extensions in the sense that those dependencies / libraries are useless without quarkus.
Exactly that. E.g. in case of Hibernate ORM, the corresponding Quarkus extension will take care of having the persistence.xml being parsed at build time, registering entity properties for reflection in native binaries, making sure all needed resources are added to native binaries, etc.
They also curate the right set of matching (transitive) dependencies for each library, you just need to add the Hibernate ORM Quarkus extension, without having to think too much about versioning. Also they integrate different APIs: e.g. when having the Kafka Streams extension and the MicroProfile Metrics one, you'll automatically get metrics for your streaming pipeline.
Disclaimer: Working for Red Hat, contributing to Quarkus a bit (Kafka Streams and Debezium outbox pattern extensions).
Don't Java folks have Gradle/Maven already? What's the difference between Quarkus' Hibernate Validator vs its vanilla version?