No problem. Until I tried an alternate launcher, I, too, didn't quite understand what it did.
First, think of a launcher as just another app. It doesn?t do anything to permanently change your device. You can uninstall a launcher, just like you can uninstall any app. Or, you can have a launcher installed, but simply choose not to use it, just like any other app. You can have multiple launchers, and use the one you want, depending on your mood. It's up to you.
A launcher allows you to more extensively customize your home screens and app drawer. Here are some examples that apply to many launchers, but certainly to Nova, the one I use.
The standard Android launcher gives you five home screens, and you can't increase or decrease that number. Nova lets you vary that number, and also lets you reorder which home screen goes where. I currently have nine home screens, but I could change to only one if I wanted. When I swipe to go to a new home screen, I can change the visual effects to something different - mine is kind of a 3D effect as I swipe to another screen. I can also make the home screens a continuous loop, so there is no end or beginning - I can swipe left forever and ever! I can choose to have more or fewer columns and rows available for placing shortcuts. I can rename my shortcuts, and change the shape of folders that contain multiple shortcuts. Widgets can be overlapped. The app drawer can be set up with folders and tabs. I can hide the notification bar, the persistent search bar, and the dock, as well as change the number of shortcuts in the dock, and the number of docks, which can be swiped left or right, independent of the home pages.
That's just a sample. There are a bunch of other tweaks and adjustments, way too many for me to describe. The best way to get a taste of it is to install the free or trial version of a launcher, and see for yourself. There's a little bit of a learning curve, but once you figure it out, you'll never want to go back to the stock launcher (but easily can at any time)!