What is this “link prefix” stuff in the settings?

Check out this (slightly old, but still relevant) video that will explain link prefixes in ThirstyAffiliates:

in Getting Started
[data-image-id='gourmet_bg']
[data-image-id='gourmet_bg']
[data-image-id='gourmet_bg']
[data-image-id='gourmet_bg']
[data-image-id='gourmet_bg']
[data-image-id='gourmet_bg']
[data-image-id='gourmet']
[data-image-id='gourmet']
[data-image-id='gourmet']
[data-image-id='gourmet']
[data-image-id='gourmet']
[data-image-id='gourmet']
[data-image-id='gourmet']
[data-image-id='gourmet']
[data-image-id='gourmet']
[data-image-id='gourmet']