Anything that's prenuptial happens before two people get married. A common prenuptial superstition is that a couple shouldn't see each other on the day of their wedding until they're actually walking down the aisle.
You're most likely to find this adjective in the legal term "prenuptial agreement," or prenup for short. This friendly-sounding phrase describes a legal contract signed by two people before their marriage, in which they agree to certain terms if they eventually get divorced. A prenuptial agreement might include details of dividing money or property, for example. Pre- means "before," and nuptial comes from the Latin nuptiae, "wedding."