Please refer to these two pics. There's this blue line next to the line number in my code. Hovering over it selects the code. I am not sure how it got there and how to get rid of it.
Clicking that line shows this:
EDIT: for whoever closed this question as "general computing hardware and software on Stack Overflow": this is not that. Plus there are several Xcode UI questions on Stack Overflow:
For example:
How to hide Blue line covering views in xib/Storyboard in Xcode 7.2
How to get rid of blue borders around transparent objects in XCode