Some Reddit users have noticed that they can't sign-in to their accounts anymore on mobile, as the option to do so has been removed for them. Reddit has made it difficult in the past for users to access the site on mobile in browsers.
Reddit is running an experiment on the site currently that limits mobile access to some users. These users notice that they can no longer sign-in to the site using a mobile browser. Reddit asks them to use the official application on mobile to access the site while signed-in to their account.
Signing-in gives Reddit users access to their history on the site, to bookmarks, and the ability to interact on the site actively. They may post new threads or comments, or communicate with other users directly among other things.
Reddit is interested in getting users to use its application on mobile. Use of a dedicated mobile app gives the company access to valuable data, more control, and the ability to display advertisement to users.