'document.write can be a form of eval/evil'

I have just completed the 'Check for Palindromes' task on FreeCodeCamp (yay me!) and although my code was accepted, I wondered why there are those irritating little exclamation marks telling me that 'document.write can be a form of eval'. If possible, could you provide me with a brief explanation of why this is the case, how it works and what I should be using instead?
Thanks!