The Play Browser is an in-game list of all uploaded levels.
The Newest Additions is basically any new levels that have been made in the past 2 weeks. Here you can find fresh new designers or old-time favorites.
Friends Levels is a function in the Play Browser that allows you to "subscribe" to someone, so their latest levels appear in the "Friend's levels" category in the Play Browser.
In the current version of Atmosphir, all you must do is add a friend on the official website to see their levels in the filter.
My Levels are where you can view your personal creations.
The LOTD (Level of the Day) Filter is where you can view the Level of the Day and past LOTD's as well.