go in one ear and out the other

If you say that something you hear goes in one ear and out the other, you mean you quickly forget it.

左耳朵进右耳朵出;当作耳边风

If I have to listen to something I don't understand, it just goes in one ear and out the other.

要是非得让我听我不懂的事情，那也只是左耳朵进右耳朵出。