In official terms, Name ID is a SAML concept which identifies the subject to whom the SAML Assertion relates to.
To give more context, NameID contains several attributes. The most important ones are its VALUE (example: email@example.com) and the format in which it's delivered.
NameIDFormat is used so that the Service Provider doesn't have to guess how to utilise the value of NameID.
>> Hello Application, here is (firstname.lastname@example.org) as a user identifier, it is in email format, good day!