if I know nothing about Android, is it crazy to have a rooted phone?
If it's rooted, leave it be. It's very unlikely for the S4 to receive many (if any) more Android OS updates officially since it's now a couple of generations old. I use a rooted S4 myself, and love the fact that I can remove bloatware at will and make backups of everything. 

With that being said, some more info from your device might help us direct you to info on your particular ROM if you have one, and how to lock it back down if you still so desire. Go into Settings>About Phone and see what Android version you're running and the build number for starters. All of those scary warnings in forums are there for a reason: with root you have unrestrained access to the very innards of your phone. If you start changing things in certain areas you can mess things up. However, it generally takes more than doing everyday tasks to do it. 

Armed with the information from the About Phone section, head over to the XDA Forums to find out just about anything you'd ever want to know about that phone, along with some generally very knowledgeable and helpful people.