A huisnummer is a house number.
Modern Dutch addresses are composed of a street name, house number, postal code, and place name. The house number follows the street name, e.g. “Dorpsstraat 15.” House numbers could change over time.
Such addresses with street names and house numbers are a relatively modern invention. In the 1800s and in some places into the 1900s, houses were not numbered per street but by ward. Before 1800, there is a great regional variation. Some places numbered houses, while in others, houses were just indicated by their owners, or by the name of the house or sign.
Please note that house numbers are not the same as cadastral numbers. See 7 Tips for Finding the Address of Your Dutch Ancestors for more suggestions.