Onyx for Mac is a popular free utility that’s been available since 2003, and is regularly updated by Titanium software. It lets you verify the structure of your Mac’s hard drive, and perform a number of other functions in a fairly simple way. You can empty different caches, rebuild services and indexes, and loads more.
It’s not for everyone, but if you have a reasonably good understanding of how your Mac works, and want a good free tool to do some occasional cleaning up on your Mac, it’s a great tool. While I would not recommend it for complete beginners, if you’re an intermediate Mac user and would like a free maintenance tool in your arsenal, this is probably the one I would recommend.
There are different versions of Onyx specific to each macOS / OSX operating systems. From the Titanium software homepage, you can find the version that’s right for you. You can download Onyx from their homepage.
The only thing I would add is that if there are options that you don’t understand, don’t click on them until you learn more. While you’re not going to break anything, you could end up with some unintended consequences if you just click on everything. Happy cleaning !