WikiTree uses the concept Six degrees of separation to determine if a profile is connected to the tree.
Anchor/featured profiles are chosen to verify the tree connection. A selected profile is compared to an anchor profile by checking for connections (parent-child; marriage; sibling; etc.) between the 2 profiles. Each connected profile between those 2 profiles in question is called a degree.
Degrees can be genetic (blood relation) or non-genetic (in general, legal/common-law connections, such as marriage or adoption).
edit: added link for anchor/featured profiles