What is a login wall?
login wall (plural login walls) (informal, Internet) A website notification that prevents an unregistered visitor from viewing any content until signing up or opening an account.
What are the pros and cons of having users log in to the app every time?
The pros are that they'll interact more with your site, the cons are that anyone with access to the computer can act as that person and can commit fraud. If your service is interaction based, and low fraud risk, keeping users logged in might make sense (Amazon, Stack Exchange).