"I believe by all 6 characters he means all 6 classes." - Ahh... indeed, my bad.
Ladder characters are pretty much any characters created after the 1.10 patch. You can still make new Non-Ladder characters in 1.10 though, but they can't get certain items, create certain Rune words, etc. Any characters created before the 1.10 patch are Non-Ladder and can't play in the same games as Ladder characters. That's the basic gist of it anyways.