虹ヶ咲 is short for ラブライブ！虹ヶ咲学園スクールアイドル同好会, or Love Live! Nijigasaki High School Idol Club. It’s a multimedia project containing anime, game, live (in real person), etc.

As of now the 2nd season of the anime is on air in Japan. Viewers usually discuss about the current episode under the tag of #虹ヶ咲, which is quite common for anime viewers or more generally TV viewers.

Learn more about Nijigasaki High School Idol Club on wikipedia, fandom, or the official website.