What is Azure Service Principal

Question

My requirement is simple. I want to login to Azure through my shell script in non-interactive mode, but "az login -u username -p password" command gives the following error:

Get Token request returned http error: 400 and server response: {"error":"invalid_grant","error_description":"AADSTS70002: Error validating credentials. : SAML token is invalid. : The element with ID 'xxxxxx' was either unsigned or the signature was invalid.

Some site told me to create a service principal. Now my question is, what is a service principal, and how do I create a service principal so that I can execute my commands (for creating different resources like app gateway) from my shell script?