It's a common shortening, particularly in contexts related to medieval history or literature, where "Fra" is used to refer to a friar, especially a Franciscan friar.
For example:
* Fra Angelico: A famous Italian painter who was a Dominican friar.
* Fra Bartolomeo: Another famous Italian painter who was a Dominican friar.
So, when you see "Fra" in these contexts, you can safely assume it stands for "Friar."