First of all, this is all predicated on you wanting to switch which computer you have Houdini installed on (or if you want to reformat/reinstall your operating system). If you don't need to do that, returning licenses doesn't matter to you.
To switch which computer you have your licenses installed on, you have to “return” them to the Side Effects Software license server. (You can only do this a limited number of times, usually 3.) When you return them, you create what's called an “entitlement” on our license server. When you use the Houdini License Administrator to install licenses from the license server, you are “redeeming” entitlements.
So, to switch which computer your licenses are installed on, you first use the Houdini License Administrator to return your licenses (by right-clicking on the license, selecting Return License, and logging in using your username and password), then do whatever changes you need to do (for example, reformatting your computer, or setting up your new computer). Then, after you install Houdini again, you can redeem those licenses using File > Install Licenses in the Houdini License Administrator.
Before 9.1.140, it wasn't possible to do this with all the licenses you get when you buy Houdini Apprentice HD. With a build >= 9.1.140, it is.
I hope this clears things up!