From the help section:
Channel, then click to select an option, such as Signal Cutoff, Band, Channel, or Floors. This overlay identifies regions covered by specific channels, or regions with overlapping coverage on one selected channel or all channels in the 2.4 Ghz or 5 Ghz radio band. Hover your mouse over coverage areas for details about the APs. |
What this overlay helps you to understand is at what locations your have Co-Channel-Interference (CCI), or where you are in the reach of multiple APs that are transmitting on the same channel. You can set the Signal-Cutoff to control what signal strength is considered overlapping. In short, in the marked areas in your screenshot, at those locations the client can see two or more Access Points transmitting on the same channel.
In WiFi design, in general, you try to reduce the CCI, so try to keep this overlay as empty as possible while keeping the overall coverage (all channels combined) as good as possible.