Power cycling means turning the Kobo completely off by long pressing the power button. You’ve already done that before, so now you know the geek term for the process.
Usually, I wait ten seconds before powering the Kobo back on, but some ppl wait thirty seconds.
Signing out and signing back in means to go to More>Settings>Accounts>Sign out. This will remove your database from the Kobo, then reinstall it when you sign back in. Database corruption can happen if you’re not careful when ejecting from your computer, but there’s also a database corruption bug out there that some ppl encounter.
Sometimes you can try More>Settings>Device information>Advanced>Repair. This usually fixes database errors with purchased books and library loans.
You asked us to describe the help as if you were five. If my explanation is still too technical, just let us know where you’re getting confused. More information helps us help you better. And don’t worry. You’ll be up to speed in no time.