A familiar plural 'you' used to address two or more people. Common in casual or male speech, though sometimes used by young women colloquially.
君たち、宿題はもう終わったの？
Have you guys finished the homework yet?
君たち、ここに並んで待ってください。
You all, please line up and wait here.
君たちのことは信頼しているよ。
I trust you guys.
友達どうしなら『君たち』で十分通じる場合が多い。
Among friends, 'kimitachi' often works fine.