It means that a Dutch court can't test the constitutionality of laws made by the Dutch government in concert with parliament.
In legal parlance translated from Dutch, these would be called "laws in the formal sense."
The way it works is that the Dutch government (i.e., the Dutch ministers and the King, albeit the latter only in a ceremonious role) proposes a bill, and the two Dutch legislative houses (House of Representatives and Senate) vote to pass it.
A law that is a product of this process is deemed a "formal law" or "law in the formal sense." Courts cannot test these against the Dutch constitution (i.e., look to see if they're constitutional).
Other forms of legislation can be tested against the constitution by courts.
These are called "material laws" or "laws in the material senses" because, materially, they function as a law in the sense that they prescribe something and are generally binding. Still, they haven't been established in a "formal" manner through the process I described above that involves the government and parliament.
Examples of such material laws are municipal ordinances and royal decrees issued by the Dutch government (akin to the American executive orders by the US president).
Some laws that have been materialized through the process described above are also considered material laws instead of formal laws, but that's more a matter of exception when they don't have a generally binding character for all citizens (e.g., a permission law for the marriage of a specific member of the royal family).
A judge can't look at these formal laws and rule that they're unconstitutional.
Ironically, the Dutch constitution itself (art. 120) prohibits this test.
The logic at the time was that they wanted to prevent the judicial branch from second-guessing the legislative branch and that if it misbehaved, the voters could punish them during the next election round.
Additionally, they wanted to enshrine that the government, in concert with the two legislative houses, should be the unimpeachable sole authority to create laws.
However, this means that the Dutch constitution functions more as a set of guidelines for the highest level of legislators than a strict set of rules to abide by.
That said, nowadays, there is some political will here and there every couple of legislative sessions to reform it so that the courts are allowed to test against the constitution, with some ideas even going as far as establishing a formal constitutional court for this purpose.
As someone who used to practice there, I think it's more a matter of trivia that raises eyebrows in your first year of law school than something with many practical consequences.
As stated before, international and EU treaties have taken over the Dutch constitution's role in keeping the legislator in check. So far, legislators haven't sought to cross the lines in remarkable ways.
Nevertheless, I'd welcome testing constitutionality as an extra layer in the legal firewall, provided it's designed in a way that leads to results seen in the German, French, and Scandinavian models, as opposed to the results and effects caused by the SCOTUS in the US.
What does that mean in practice? That the constitution always overrides any law passed by parliament?