After Russian drones were downed in Poland, what does triggering NATO Article 4 mean?

Poland has triggered Article 4 of the North Atlantic Treaty Organization, NATO, after the country reported 19 “breaches” of its airspace by Russian drones on Wednesday.

​ Poland has triggered Article 4 of the North Atlantic Treaty Organization, NATO, after the country reported 19 “breaches” of its airspace by Russian drones on Wednesday. Read More Breaking News, Latest News and Videos | CNN