Playtime is a VST instrument for the Digital Audio Workstation REAPER and works in REAPER only. It is designed to make loop-based music production in REAPER easier and much more fun. In particular, it offers a Session View loosely inspired by the one in Ableton Live. That means it enables you to play loops in a very intuitive way - just by triggering them using your MIDI controller or the graphical user interface.
This playful way of handling loops can be useful in many ways. For example, you can use it as a live performance instrument or for sketching your arrangement. Playtime integrates with REAPER by leveraging its unique extension API and thereby aims at a better user experience than combining REAPER with standalone software using ReWire. It essentially works by positioning items on tracks in realtime.