What it is
Resolves the domain through more than ten public DNS resolvers that filter content: ad and tracker blockers, malware filters, and family or parental control services. Each either returns the real address or a blocked response.
Why it matters
Being blocked by a filtering resolver is not the same as being malicious, but it does mean a slice of users cannot reach the site at all. Owners rarely notice, because the block is invisible from the inside. Which categories of filter object is a rough guide to what the domain is thought to be.