The Most Active Prospects widget in the My Performance Q page shows your most active prospects for the selected time period. The bar height represents the total messages in that period between you and all of the participants from that domain.
This widget is a graphical representation of the interest of the different businesses you're communicating with. The most active prospect could be the business that's most interested in what you're offering while the least active prospect is the contrary. Based on this, you can evaluate which businesses you need to put more focus on.