Get your head round something - Idioms by The Free Dictionary
get your head round something
get (one's) head around (something)
To understand something that is challenging or confusing. It took a while, but I've finally gotten my head around this chapter in my calculus textbook. Wait, you two are dating now? It's going to take a little while for me to get my head around that!
get your head round (or around) something understand or come to terms with something. informal
get your ˈhead round something (British English, informal) understand something difficult, often with a lot of effort: The plan is so complicated — I’m still trying to get my head round it.