What does 2 lines mean?

Answer 1

It's reserved for apartment, suite and box numbers.

For instance, if your address is:

14925 North Archibald Lane

Apartment 205

Or

2609 East Rosa Parks Blvd

Suite 400

It makes sorting and delivery easier for the post service that handles your mail or packages, thereby enabling them to get it to you faster.

Even if the whole thing fits on one line, it's best to use both because, as mentioned, it helps reading and sorting.