Che cosa significa with people like that.who knows?

https://youtu.be/qmU_pN0bEn8 ?

Vedi una traduzione

It means that the person they are talking about is unpredictable in behaviour.

In the video you linked the conversation was basically this:

(talking about a rude woman jumping the queue)
man: "the rude woman left. she must have been embarrassed, and her kids were upset."
girl: "yea, well she probably learned her lesson".
man: "I hope so, but with people like that, who knows?"

He means that the woman knew there was a queue, but she didn't care, and a normal person would learn to be more considerate of others and not cut into queues anymore. But rude people such as the woman can not be predicted to behave like a normal person. Maybe she will be more polite, or maybe she will remain rude and keep cutting in queues. Who knows? no one can predict it.