In drawing a tiger, you show its skin, but not its bones; in knowing a man, you may know his face, but not his heart.

解释 ( paraphrase )
画一只老虎，容易画出它的皮毛却很难画出它的骨头；认识一个人，容易了解他的外表去却很难了解他的内心。

这条谚语告诉我们：了解一个人很不容易。特别是对那种表面做一套、内心又是一套的人，应该处处提防，以免受骗上当。

This proverb points out that it is difficult to get to know waht a person is really like.

面：面貌，表面。
      Face.

同义词 ( synonym )
金玉其外，败絮其中

造句 ( example )
“知人知面不知心”啊!从外表根本看不出来他会是残忍的凶手。