I looked at the definition of dependent contractor "a contractor that is economically reliant is one principal" what does this mean? By principal, do they mean the head of the company? And if so, wouldn't that just be an employee.

The term "principal" here means "client." And I think what you meant to write is that a dependent contractor is one "that is economically reliant on one principal." This means that as a contractor the majority of your business comes from a single client, and that the client dictates certain aspects of how you deliver your services. For example they may dictate hours that you work, or that you wear a company uniform, or what tools you use. Often a dependent contractor will have no hard end-date in his/her contract.