davy hit it right. In Italian ossia means "or", "otherwise" , "or else", all meaning an alternative way of playing a passage. I'm glad that composers include these options. For example, what would we do without the ossia in Rachmaninoff's big cadenza in the first movement of the third concerto? Most, but not all, artists opt for the ossia, not the more straightforward version there.
Most codas consist of a few measures added to the end of a piece to create a more effective termination. Without the coda, many a piece would end on a "weak note". Some of the most lovely codas I've ever played appear in the Rachmaninoff Preludes, for example.