Affinity means this person has recently shared a post of yours.
Tribes is based on the principle of reciprocity.
When you join a tribe, your tribemates support you by reading, commenting and sharing your content. In turn, you do the same. Now you don’t have to share every post your tribemates write, but it’s only right that every member of the tribe makes a concerted effort to participate.
When a tribemate shares your post, Triberr labels their post as an “affinity” post, and moves it to the top of your stream.
If you don’t want to see posts from people who share you content on top, you can disable this feature by going to Account Settings -> Tribal Stream and disabling the option.