Ok, you probably know this, but I'll go over it just in case. Yahoo finance is s service provided by Yahoo. If you don't already have one, you'll need a Yahoo account to use it. I'd go to yahoo.finance in Safari first to set it up and see if it works. When you've done that you should be able to use the same account and password in the app. The app may ask to send you a security code to activate. That is usualy send to your phone, but there are other methods.
This advice is not 100% guaranteed. I went to the Yahoo site, checked out the finance page, and guesstimated the rest based on how most other services of this type work.
The Yahoo Finance app is probably just the app from of their website. It may or may not work better for you than just going to the site using Safari. Depends on how much work they put into the app, and you own tastes and preferences.
I can't say/guess much more since I don't use the service.