Mathematica allows you to make assignments that override the standard operation and meaning of built-in Mathematica objects.
To make it difficult to make such assignments by mistake, most built-in Mathematica objects have the attribute Protected. If you want to make an assignment for a built-in object, you must first remove this attribute. You can do this by calling the function Unprotect.
There are a few fundamental Mathematica objects to which you absolutely cannot assign your own values. These objects carry the attribute Locked, as well as Protected. The Locked attribute prevents you from changing any of the attributes, and thus from removing the Protected attribute.