These days the standard way to get your java libraries is through Maven. However, sometimes you still need to deal with jar files that aren’t available through Maven at all. Perhaps some software vendor has provided a set of jars, or you have a custom build that you aren’t allowed to publish at all.
If this is the case, you could just reference the jars on the file system, the old school approach. But you can also mavenize them yourself, and store them in your local Maven repository (which can be found in your user profile folder, in the folder .m2). That makes it possible to reference these libraries in a pom.xml or in a build.gradle file, giving you all the advantages of modern dependency management.