An echo chamber is a metaphor for a situation in which believes are amplified or reinforced by communication or repetition inside a closed system (Wiki-article).
Echo chambers can be quite common on social medias. If you here join a group of some political agenda, you are likely to meet only people in here with an opinion that conforms to this agenda. Since it can be daring to utter critique in such an environment, the group can end up confirming this agenda and nothing else. Hence an echo chamber where nothing opposing enters.