The Application Source is the most recent way the candidate was referred to the job they applied for. If there is no referral id from the site they used (such as Indeed), then whatever they choose from the "How did you first hear about this opportunity?" section will display. (If you have questions about editing that dropdown list, you can find instructions on how to do so here.
The Candidate Source would be the original source of the candidate. The application source may be different with future applications from that candidate - i.e. they first applied from a job on ZipRecruiter, but then they found a job on Neuvoo a couple weeks later - Candidate Source would be Zip, application source for the second app would be Neuvoo.
The Application Method is how the application was processed into the system. Did it come through Apply with Zip, Apply with Indeed, etc.
Example: A candidate finds a job on Indeed and clicks through to your Career Portal – the source is now Indeed. If the candidate then uses the apply with Indeed button, the method is apply-with-Indeed. If they use your application on your Career Portal (with no integration) then the method is job-board.