To me it is a collection of tools to simplify channel management. It adds different buttons, and different tools on your YouTube pages.
Much of what it does can be done without TubeBuddy - but would be a lot more work. Like tag & keyword research. You can do it through Google but this simplifies (and speeds up) the process a lot! Other tools, like comment sorting are things that YouTube should do, but does not. The comment sorting (needs reply, contains question etc) is one of my favorite time saving tools.
Whether it is worth the money is up to you. You can try the free level and see what it helps you with. If you need more... there are different subscription levels.
Using it will teach you a lot about tag/title/descriptions and searchability. It can help you get your video searched out, and hopefully clicked on. What it will not do is make you content any better ha ha... bummer...
@Tito Tim put it well, the best way to describe TubeBuddy is a swiss army knife of tools that can help you manage your Youtube channel from the upload of videos to the comments on the videos. I would suggest checking out their channel for videos showing you all the features in detail.