This feature is called brace expansion.
Generally you can write file{1,2,3}
and bash expands it to file1 file2 file3
before running the command.
If you write
mkdir foo{1,2,3}{a,b}
it will be equivalent to
mkdir foo1a foo1b foo2a foo2b foo3a foo3b
and will create those 6 directories.
In your case ({,}
) you are adding nothing and nothing and therefore get the same word twice.