Get One's Head Around Meaning

(idiomatic, transitive) To understand; fathom; solve.

Example: 2009, Wahida Shaffi, Our Stories, Our Lives: Inspiring Muslim Women's Voices, ISBN 9781847424105:
  When my mum did pass away I fell out with God because I couldn't get my head around why he'd taken away the one person that we needed in our lives, because dad was no good to us.