To "get (one's) head around" something means to understand something or figure something out. Here are a few examples:
"I gave my girlfriend flowers, chocolate, and jewelry for Valentine's Day, but she didn't like any of it. I can't get my head around what exactly it is that she wants."
"I don't know if I can get my head around what my boss expects of me. He's asking me to be in two places at once."
"This assignment is too hard. I just can't get my head around all these mathematical equations."
I hope this helps. Please let me know if you have any more questions =)