It's an app that allows you to play rpgmaker, renpy, html, tyranobuilder & flash games on your phone.
The latest version can be found here if you are interested:
mega.nz
You just need to install the apk, make sure to unzip the Night Market full game download, hit the add game button on the joiplay app upon launching it, you'll be prompted to select a download, ie, an exe or html, select the html from the Night Market folder, name the game whatever you want, the version can also be whatever you want, lastly, you can choose a png or jpg for the the cover of the game you're adding, after this you can launch the game!
EDIT: For anyone interested, here are the links for the latest versions of the plug-ins for Joiplay, all apks--
Renpy(for games made with Renpy, visual novels usually):
mega.nz
RPGMaker(for games made with rpgmaker):
mega.nz
Ruffle(for Flash games):
mega.nz
Tyranobuilder & Html games don't rely on/require plug-ins, just the Joiplay app itself.
EDIT: Updated to the most recent versions. Have fun, everyone!