A: Co-channel interference occurs between two access points (APs) that are on the same frequency channel. The reason that you should care is that co-channel interference can severely affect the performance of your wireless LAN (WLAN).
The spectrum that's available for the deployment of WiFi is limited. For example, in the 2.4GHz band, there is just 79MHz of spectrum in the United States. Given that your IEEE 802.11n devices use a 20MHz channel, you have room for three non-overlapping channels. You'll need to reuse the frequency channels when you deploy your APs.
Co-channel interference is more problematic when you deploy your WLAN to support voice or RFID location tagging. These types of scenarios require a denser deployment of APs. Denser deployments mean that your APs are closer together. And that creates a greater potential of two devices that transmit on the same frequency channel will be close enough to cause significant interference to each other's signals.
When deploying WLAN for voice or location services, you need to balance the need to overlap your cell coverage to ensure that voice and location services don't experience gaps in service with the need to physically separate APs that operate on the same frequency.