Refers to a man's previous wife, either divorced or deceased. The context determines whether it means 'former' or 'late'.
He has two children with his former wife.
After his late wife passed away, he has been living alone.
先妻 is the man's first or former wife, while 後妻 is the wife he married after that marriage ended.
Compound of 先 (previous, earlier) and 妻 (wife).