Hi am running my first campaign using the D&D beyond app.

But I can't see any how one could view the campaign description when using the app, like with your phone or tablet.

Is it possible or do you have to use website version for that?

It would not be a problem but I have a some info for players about character creation that they should adhere to.

This should be an option. I also look for this commonsense feature of being able to read the campaign description in the app.

The app is not designed to be the entire website.  For things like that, you have to use the website.