MacOS has this terrible habit of littering filesystems with ._ files (for the search engines: “dot underscore files”). Apparently Apple uses these files for some sort of extended metadata, extending Apple’s track record of overcomplicating file systems that goes back to the bad old days of Resource Forks. (Remember BinHex? Good times.)
OK, to be fair, having some out of the way place to put extra data around files is sometimes useful. What I hate is the way the ._ garbage breaks other tools. My latest example is rsync –delete; it won’t delete a directory with a ._ file in it. Somehow in my trying to sync up directories between two Macs via a USB disk I’ll end up with some stray ._ files that then escalate to whole stray directories full of ._ files.