Because your website gets all possible traffic from that keyword, your Visibility is 100%.
This metric in Ahrefs’ Rank Tracker shows what percentage of all clicks for the tracked keywords land on your website.
Visibility in Ahrefs' Rank Tracker
For example, let’s say you track one keyword, and all the top positions in Google are occupied by pages on your website. Because your website gets all possible traffic from that keyword, your Visibility is 100%.