Updated June 10th, 2019.
Most external hard drives come formatted for use with PCs. The usual PC format these days is known as FAT32, where FAT stands for “File Allocation Table” and 32 means that 32-bits have been allocated to identifying each “cluster” of data. Interesting indeed. The main thing is, FAT32 disks will work on a Mac, but slowly. In fact, VERY slowly. You want to avoid using the FAT32 unless you’re going to move that disk between a Mac and a PC. If you’re going to do that, leave the disk as FAT32. Otherwise, the format you want, for a Mac, is Mac OS Extended (Journaled).
The good news is you can reformat a FAT32 disk as Mac OS Extended (Journaled). You’ll do that with Disk Utility, which came with your Mac.
Here’s File/Get Info for a FAT32 disk.
Here’s File/Get Info for the same disk after reformatting as a Mac OS Extended (Journaled) disk.
Note: reformatting a disk will ERASE it. If you have a FAT32 disk with stuff on it, copy it to some other disk, then reformat (use Disk Utility), and then copy your stuff back.