Video: Understanding Constraints
Constraints are non-view specific elements that can function independently of dimensions. Constraint elements appear as blue dashed lines in all views in which their references are visible; dimensions are view-specific. You can modify and delete constraints independently of dimensions or remove them when you delete dimensions.
In Revit, elements can reference each other in many ways, including the following:
These explicit and implied constraints require that each element knows about the other. That is, one element must reference the other element.