This is a more technical post, so you can just skip it if it’s not your bag.
All of the data on your Mac’s drive has to be organized in some fashion, so that you can get to it. This is accomplished via what is known as a file system, and the only metaphor I can come up with for it is increasingly obsolete: the card catalog of a library. The library has all these books, but if there weren’t a means of knowing which ones are where, they wouldn’t be of much value.
When you talk about formatting a disk, what you’re talking about, at least in part, is establishing a particular file system on it. Once that’s done, what you’ve got is called a volume, which is where all your information gets stored. (And, though unusual, a disk can also be partitioned, allowing it to appear it were multiple disks, each with its own file system.)
There are many different kinds of file systems for computers, some old, some new. Macs are partially compatible with some of the ones commonly found on Windows computers, so you can work with those disks. But Macs are most reliable when using Apple’s own file systems, which is why it’s usually advisable to reformat an external drive that’s going to be solely used with a Mac, because it probably came preformatted for Windows. You do this using the Mac application Disk Utility, found in the Utilities folder of Applications.
For about three decades, Apple’s primary file system has been called “Mac OS Extended”, also referred to as HFS+, or sometimes just HFS (though that’s not really correct, as it was the name of its incompatible predecessor found in 80’s and 90’s versions of Mac OS).
Starting with macOS 10.13 High Sierra, Apple has introduced a new file system, simply called APFS, because they felt HFS+ was no longer capable of supporting things they wanted macOS (and iOS) to do.
APFS was designed around solid state drives (rather than spinning hard drives) from day one. Interestingly, it doesn’t necessarily perform better than SSD’s do under HFS+, but it takes advantage of how SSD technology works to do things that aren’t as available with hard drives. A hard drive has to physically move a “head” (like a turntable arm) to a specific physical location on a disc. This means that if you ask too many things of a hard drive at once, it gets pulled in multiple directions, dramatically slowing things down. An SSD, being just chips, has no such physical constraints, and can respond to simultaneous requests for data with ease. The downside is that APFS performs very poorly when used with hard drives, as anyone with a hard-drive based Mac (e.g. many configurations of iMac, Mac mini, and MacBook Pro with optical drive) who has upgraded to Mojave or Catalina, both of which require APFS, can tell you.
The fact that you transitioned from HFS+ to APFS without your noticing (that is, assuming you were using an SSD), during a macOS upgrade, is a testament to Apple’s engineering. You shouldn’t have to be aware of how your Mac stores your data on its drive.
I’ll talk about more of what APFS can do in another post.