Editing CSS in wordpress editor

I am unfamiliar with editing code but have followed the directions on here that you have given others and deleted a few lines of code (the lines of code that had to do with media grid, post masonry grid…) I went back into the editor to add in a line of code that you suggested for not showing page titles on any page, and it says that there are errors throughout, in places I have not touched. What happened? What can I do?