The -p flag will create nested directories, but only if they dont exist already.
For example, suppose you have a directory /foo that you have write permissions for.
mkdir -p /foo/bar/baz # creates bar and baz within bar under existing /foo
It is also an idempotent operation, because if you run the same command over again, you will not get an error, but nothing will be created.

mkdir newfolder #Creates new folder in current directory
mkdir -p newfolder/subfolder #Creates folder with nested subfolders