Interested in Language
What does "They can't get their heads around it" mean?
Is it that they can't "understand" something?
Yes, that's right. Definition from Cambridge Idioms.
get your head around sth informal
to be able to understand something
He's tried to explain the rules of the game dozens of times but I just can't get my head around them. [usually negative]
Thanks for helping me get my head around it