As I know, you can enter the meta tag description and the title when creating a new hub.
Meta tag title is the same as the title of your article so it is important to put your keywords there. If possible, limit the length of the title to 65 characters because Google shows only first 65 characters in the search results.
The description should be no more than about 130 characters (about 1 or 2 sentences). Again, use your keywords there.