Coverage defines the ability of wireless clients to connect to a wireless AP with a signal strength and quality high enough to overcome the effects of RF interference. The edge of the coverage for an AP is based on the signal strength and SNR measured as the client device moves away from the AP.
The signal strength required for good coverage varies dependent on the specific type of client devices and applications on the network.
To accommodate the requirement to support wireless Voice over IP (VoIP), refer to the RF guidelines specified in the Cisco 7925G Wireless IP Phone Deployment Guide. The minimum recommended wireless signal strength for voice applications is -67 dBm and the minimum SNR is 25 dB.
The first step in the analysis of a post site survey is to verify the 'Signal Coverage'. The signal coverage is measured in dBm. You can adjust the color-coded signal gauge to your minimum-allowed signal level to view areas where there are sufficient and insufficient coverage. The example in Figure 8 shows blue, green, and yellow areas in the map have signal coverage at -67 dBm or better. The areas in grey on the coverage maps have deficient coverage. Source from Cisco
https://www.cisco.com/c/en/us/td/docs/wireless/technology/vowlan/troubleshooting/vowlan_troubleshoot/8_Site_Survey_RF_Design_Valid.html