A huwelijkscontract (literally: marriage contract) is a prenuptial agreement. Sometimes, they are called huwelijkse voorwaarden (conditions of marriage).
Whether a couple had a prenuptial agreement depends on their social status. Richer families were more likely to draw up a marriage contract than poorer families. It is also more common to find a marriage contract if one of the spouses is widowed with children, in which case the marriage contract may make provisions for the children. Whether there is a marriage contract also depends on the time and place, since in some areas, like Groningen, marriage contracts were more common than in other places.
Often, the marriage contract includes the last will for the spouses, stipulating who inherits what if they die. The marriage contract will often also name guardians of any children of the marriage. This can give you the names of other associates. Marriage contracts will also give you some idea of the economic situation of the couple, and will tell you if they were literate or not.
Marriage records were recorded by notaries. Before 1811, not all areas had notaries so you can then find them in the series of voluntary court records. The marriage contract is typically drawn up in the period between the first marriage banns and the actual marriage.