To use this app you must install an OEX compatible GUI app. some good apps are already available on the play store

Chessis
Chess For All
Chess Pgn Master

This app is completely free and doesn't show any ads!

Stockfish is an open-source project which developed by Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad, Stéphane Nicolet, Stefan Geschwentner, and Joost VandeVondele, with many contributions from a community of open-source developers.

you can read more about Stockfish at www.stockfishchess.org