foo() {
        for arg in "$@" ; do
            echo "arg is \"${arg}\""
        done
    }

    foo 'bar baz' 'spaces in filename.txt'

When writing shell scripts it's a good idea to use the -- option whenever possible

    stupid_backup() {
        cp -a -- "$@" /stupid/backup/dir/
    }
 
    # copies 2 files
    stupid_backup --files '-with -leading -dashes'