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While the specific command(s) you run using a for loop may be different from the above example, all for loops adhere to this basic structure.

MacOS: Hidden .DS_Store and ._AppleDouble files

MacOS creates, among other hidden files, .DS_Store files and sometimes "AppleDouble" files on drives or network shares that are not formatted with an Apple file system.

.DS_Store files are created by MacOS Finder and store metadata about folders, including for instance Finder window size/position/configuration. Apple used to provide a technical bulletin to explain the use of these files and how to prevent them from being created on network shares, but the support document has been removed. These files typically can be safely removed.

"AppleDouble" files appear to be duplicates of other files existing in the same folder, prepended with dot and underscore (._). These files contain so-called Resource forks of a file, which are saved as a separate file on non-Apple file systems: https://en.wikipedia.org/wiki/AppleSingle_and_AppleDouble_formats. Exercise care when working with these files, as they might contain metadata that is not included in the Data fork portion of a file.