It is just their last name. By the way, the "di" should not be capitalized.
"See you tomorrow!" is a common English equivalent of "A domani!"Specifically, the preposition "a" means "to, until." The masculine noun "domani" means "tomorrow." Its singular definite article is...
it means see you tomorrow.
In italian, generally, di means of.
di dove sei (not the number six because the tu for of essere is sei. the number six is spelled the same way.) means where are you from.