A corpira was a unit of extra-terrestrial currency. Although it was unclear where it was legal tender, Turlough had somehow come by two two-corpira pieces, which he used as a way to determine his fate. On Frontios, he secreted them into both hands, and then had Norna choose which hand it was in. When she guessed correctly, he gave her the piece and then, somewhat reluctantly, set off to find the Fifth Doctor in the dangerous, Tractator-made cave system underneath the colony buildings.
Before he left, though, Norna discovered that he had actually held a coin in both hands, meaning that he had rigged the game, and probably didn't hold any superstitious beliefs about the coin's impact on his fate, after all.