Huwelijkse voorwaarden (literally: marriage terms) is the Dutch term for prenuptial agreement. Historically, like today, most people did not have huwelijkse voorwaarden. Prenuptial agreements were often made by people who were well-to-do or who had children from a previous marriage to provide for.
A prenuptial agreement identifies the spouses, sometimes accompanied by family members, and describes how the estate should be divided in case one of them dies. Usually, different scenarios are described based on who dies first, and if there are children. Sometimes the agreement describes what each spouse brings to the marriage, but usually it just says that they both bring everything they got. The prenuptial agreement is signed by both spouses, and sometimes by their kin as well. These signatures can be helpful when untangling same-named persons and family relationships.
Prenuptial agreements can be found in notarial archives or, in a time and place where there were no notaries, in the voluntary court records. It is rare to find them online, although there are several digitization projects involving notarial and court records. Check the Digital Resources Netherlands and Belgium website to find out if any notarial or court records are available online for the time and place when your ancestors were married.