What does RI stand for in king george's signature?

The RI stands for Rex Imperator.

Rex is latin for king, Imperator is latin for emperor.

The RI is most likely due to his standing as Emperor of India. Since the monarchy no longer functions as such, I future monarchs of the UK will probably just sign R at the end of their name.

This is purely speculative after digging around on the 'net after being curious myself.