I’m going to go with sleep.
Alien: So why do you do it?
Human: We don’t know. But we do it for seven or eight hours every day or we’ll die.
Alien: Sounds boring.
Human: Not always. Sometimes they show movies.
Alien: Why do they do that?
Human: Dunno. You usually don’t remember them.
Alien: Weird.
What would you choose?
Music might be one such thing. To an alien we sit there getting pleasure listening to apparently random some random noises but not others.
What a balk is.
I’d rather explain baseball than the popularity of Kim Kardashian.
Alien: So, who is the most famous human?
Human: Kim Kardashian. She makes fifty times more than the President.
Alien: Why?
If the aliens came from an extremely homogenous, cohesive species, they may have a hard time understanding our tribal nature. “So, humans tend to form into groups which disagree with other groups of humans, and each group tries its best to thwart the goals of the other group, and disavows the beliefs of the other group, even if there is scientific proof of those beliefs? And these differences go so far sometimes as to involve actual widespread killing? How have you accomplished anything as a species?”
Malort
If they don’t have laughter or something analogous, I’m not sure it would be possible to explain it.
The paramount importance of gender norms.
First thing I thought of. If they are a species capable of hearing sound, they’ll probably get our music instantly, if not they’d be stunned.
“hesitate or be unwilling to accept an idea or undertaking.”?
Or our lack of gender norms.
“What do you mean ‘gender-fluid’ On our planet, you stick with whichever of the four genders you are born with.”
See what I mean? 
What ‘icing the puck’ is.
Tripler
Hard enough 'splaining it to a human as it is. . .
The infield fly rule.
Why do American grocery stores sell chocolate rabbit ears during the spring?
Alien: “So you say that two species go on walks together?”
Human: “Yep.”
Alien: “And when the little one stops to take a shit, the big one picks it up and carries it in a bag until he can throw it away?”
Human: “That’s how it’s supposed to work.”
Alien: “And then when they get home the big one gives the little one a snack?”
Human: “usually.”
Alien: “But you are saying that the big one is the dominant species.”
Human: “Well sure. He’s the one who goes to work. Those treats sure aren’t cheap!”
Alien: “And while the dominant big one does these random tasks all day, what does the little one do?”
Human: “Sleep, mainly.”
I love SF stories where aliens can’t figure us out. One (from the 60s?) had a psychic alien’s journal with an entry to the effect of “There is this thing called LAFF that they do.I’ve never encountered a species with such joy. And the way they just empty their brains as they do it…”
Me: everyone in the US has the right to own one or more guns
Alien: guns are trophies to hang on the wall
Me: no, we shoot with them
Alien: what happens to someone or something that gets shot
Me: it gets destroyed
Alien: so guns are used to destroy other things
Me: well, only crazy people destroy things
Alien: so only crazy people have guns
Me: no. Everyone can have a gun. It’s for protection.
Alien: so it is dangerous to be a human. You must protect yourself with these guns.
Me: it’s not dangerous, most of the time. Some people like to carry them around, shoot at inanimate objects, kill animals for food, scare other people.
Alien: guns are toys that destroy other things. I understand
Porn.
The concept of “love.” Hell, we can’t even explain it adequately to ourselves.