"FIDES meant 'reliablilty' or 'good faith', a sense of trust between two parties if a relationship between them was to exist. FIDES was always reciprocal and mutual, and implied both privileges and responsibilities on both sides."
So this roughly means "good faith marriage contract". I am sure that someone who is an expert on church marriage. Latin, and legalese, will check in with a better explanation, but perhaps it refers to the marriage couple promising that there are no barriers to them getting married. I.e. no other spouses, no blood relationship, etc. A google search doesn't turn up a great definition.
Hope this helps.