expression, ichidan verb
incline one's head; tilt one's head
Describes the physical action of tilting or inclining one's head, often to show curiosity, doubt, or thoughtfulness.
彼は不思議そうに首を傾けた。
He tilted his head as if puzzled.
More common and idiomatic for expressing doubt or puzzlement. 首を傾ける is more literal and less frequently used.
Compound of 首 (neck/head) + を + 傾ける (to incline). The literal meaning is straightforward.