It's a very brief clips -- 16 seconds brief(!!) -- featuring a masked Matt Murdock (AKA Daredevil), Jessica Jones, Luke Cage, and Danny Rand (AKA Iron Fist) entering an elevator. They seem a bit agitated, which -- considering that Matt is masked -- implies that they are escaping from someone.
Anyway, Season 1 of "Defenders" appears to be debuting on Netflix on 08/18/17, if this clip is to be believed. Can't wait!!