To leave someone out of the loop; to exclude someone from a group or decision (literally, 'to place outside the mosquito net'). Often used in the passive.
重要な会議の内容について彼は蚊帳の外に置かれていた。
He was left out of the loop about the important meeting's details.
チームの決定に彼女を蚊帳の外に置くのはよくない。
It's not good to keep her out of the team's decision-making.
仲間外れにされていると感じ、いつも蚊帳の外に置かれていると言った。
Feeling left out, he said he was always left outside the loop.