Что делает касание / неприкосновение в xcode?

If a file is touched, it will be rebuilt the next time you build a target to which it belongs.

Thus, touch marks a file as being modified (and thus needing to be compiled) - easier than opening the file, adding a space, deleting the space, and then saving it.

Untouch does the opposite - marks the file as not needed to be compiled (usually a bad thing to do since the build can get out of sync with the sources, but if all you did was edit the date in a comment in a header file included by everything it allows you to avoid a full rebuild).