I moved your post back to the "Using the Community" forum as that is where it belongs. Some other Moderator mistakenly moved it to the Download & Install forum, Sorry about that.
I suspect you are asking this question because of this screen you have gotten when signing in to this forum system.
I and others have seen this same screen when click the Sign In button in Web browsers on Computers and there are 2 different conversations about it. One here in the Using the Community forum and the other in a Restricted forum that you as a normal user do not have access to.
This is some type of Mobile Device app and I have no idea WHY it is being offered to people that are signing in on desktop/notebook Computers.
For me my Default Web Browser, Chrome, saves my login ID, an Email Address, and my password for this site so when I go to one of the few restricted forums on this system I get Sign In Automatically. If I come to the site without going to one of those restricted forums I have to click the Sign In button and the normal Sign in dialog window comes up and I get Signed In by my browser.
You do not need this app and I suggest you don't download it to your phone or pad if you normally only come to this system in a Computer Web Browser.