Hi Jesús, 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 =)

Sometimes, we also say: wrap my head around. Or get my mind/wrap my mind around something.

It means to understand something properly. "I just can't get my head around these instructions" "I just can't understand these instructions"