Just wondering, because when I open the app there isn't a message or anything. I know when the App Store icon has the numbers it means there's an update, but what's it mean for other apps and where do I look in the app to find out
It will vary from App to App, but typically is an alert of some kind (Mint shows this as anotification of transactions above a certain amount). Obviously this will depend on the app and what the developers have in mind.
If in app purchases are part of the app, it may be an alert to udpates for those.
check the app for an icon that looks like a gear - this is typically a settings page, but may include links to help files. also, you can look in the app storte and see if the there is a way to contact the developer or link to a website.