This option is only available when a relation is selected to connect a contraint to. It initiates the Expression Editor and enables you to add a constraint to the knowledge base and connect it to the relation in focus.
As mentioned before, you are able to make many equal constraints with ease. However, most of of the time this is not what you want. For one thing, it will become harder and harder to know which constraint is connected to a relation (for instance, when you want to modify this constraint). Therefore, we want to stress that you should always first try to connect (and sometimes modify) existing constraints. Think about your requirements. This will save you a lot of time searching for bugs in you knowledge base.