Describes someone whose character and feelings are well known, so you can be at ease with them (e.g., 'a close, trusted friend'). Often used before a noun.
気心の知れた友人となら、何でも本音で話せる。
With a close (trusted) friend, you can talk honestly about anything.
気心の知れた仲間と週末にバーベキューをした。
I had a barbecue over the weekend with friends I was familiar and comfortable with.
彼は気心の知れた同僚だから、悩みを相談した。
He consulted a colleague he was close with about his worries.