Source defines the origin of the ticket. I believe that your query is related to the source of the incoming tickets. If the source is mail, then the requester would have raised his/her issue from their mailbox to your support address. Your ticket detail page will look like the below image, if the source is email:

If the source is portal, then the requester would have raised his/her query in your Freshdesk portal. Your ticket detail page will be like the below image, if the source is portal:

The source field is automatically populated and it cannot be changed.