A Non-Featured Merchant is a seller that is not eligible to win the Buy Box. They have been selling for less than 90 days on a particular listing, or they have not met the minimum metric requirements to be able to win the Buy Box.
To see if your competitor is a Featured or Non-Featured Merchant, you can look in Competitor Analysis for the listing. Non-Featured Merchants will have a check mark in the Non-Featured column. Featured Merchants will have no check mark.