This article will mention you what page and app variables are.
When you click variables button on top of the page as the image below, you can see your page variables and app variables
A variable has a name, a type and value. You can change the value of a variable in variable panel as the image below or flow with flow actions.
The main difference between app variable and page varible is that you can only reach page variable where you created, but you can reach app variable on every page.