Is there a word or a phrase that describes someone listening to somebody else speak to him without understanding what is being said while acting like he's getting it?
There are phrases for it. The most common one is: let it gloss over one.
My mom was talking to me, but I let it gloss right over me.
Another common phrase for it is "selective hearing" / "selective deafness."
Grandma always says my grandpa has selective hearing. He can hear just fine, but he only hears what he wants to hear.
Also, there's "tuning out."
The teacher was talking to me forever, so I just stared at her and tuned out. I have no idea what she said.
An old idiom that explains listening and not understanding is, "It was all Greek to me."
Yes, the mechanic explained it to me. I nodded and smiled and pretended to understand, but it was all Greek to me!
The best one I ever heard was:
I see your lips moving, but I don't care what you're saying.
Most people can't listen to someone speak without understanding what they're saying, not unless that person is speaking another language. People can tune out and not listen, like think about something else and pretend their listening, but that's not listening.
My mother used to tell me:
"There's a difference between hearing and listening.
If you're just using your ears, you're hearing the person, but not what they're saying. If you're listening: then you're hearing them, but also paying attention to what they have to say. So to be a good conversationalist, you should also be a good listener."
A really valuable distinction.
A really valuable lesson.