if I use git command like this:
git checkout -- .
I know its effect is to discard all unstaged files everywhere.
Can anyone tell me what is the meaning of the dot sign (.) in this command?
The dot stands for the current directory. The command you've mentioned means: Do a git checkout of the current directory (recursively). The double-dashes separate options from filespecs (like .).
The dot (.) refers to the current working directory.
You are asking git to checkout the current directory from the checked out branch. The double-dash is used to separate references, such as master or HEAD from file paths, such as myfile.cpp.
来源：https://stackoverflow.com/questions/8275452/dot-signs-meaning-in-git-checkout-command