Primary Identifiers
Whenever a Lead is collected it has at least one property that is unique to it, which can be used to identify it in a large collection of leads. It is also a value that must always be present, for every lead.
This is known as a Primary Identifier.
Setting a Field as Primary Identifier in your Lead Form Configuration means that no two leads can have the same value for that field.
This also means that this field cannot be disabled as long as the lead form is enabled.