Refers to the entrance area of a house or building, often the front door or gate. Slightly more literary or traditional than 玄関, but still common.
門口に立って客を待つ。
I stand at the doorway waiting for the guest.
彼は門口で靴を脱いだ。
He took off his shoes at the entrance.
Compound of 門 (kado, 'gate') and 口 (kuchi, 'mouth, opening'). The reading かどぐち is a straightforward kun'yomi combination.