inside; interior
(within) one's mind; one's soul; one's heart
「内面」 has another reading with different meanings.
彼女は内面がとても優しい人です。
She is a person who is very kind on the inside.
内面の美しさが大切だと思います。
I think inner beauty is important.
彼の内面を理解するのは難しいです。
It's difficult to understand his inner self.