A colloquial, often derogatory plural pronoun meaning “they” or “those guys,” used to refer to others with familiarity or contempt.
あいつら、また遅刻してきたよ。
Those guys are late again.
見てろよ、あいつらを懲らしめてやるからな。
Just you wait — I'll teach those guys a lesson.
先生、あいつらだけ宿題を忘れていました。
Teacher, only those guys forgot to do the homework.