In wireless networking, Dual Band WiFi is the capability to transmit on the 5 GHz band of 802.11a/n and also the 2.4 GHz band used by 802.11b, 802.11g, and 802.11n devices. Unlike ordinary wireless devices that only support one signal band, Dual-Band devices contain two different types of wireless radios that can support connections on both 2.4 GHz and 5 GHz links.
Dual-band devices are available as 'selectable' or 'simultaneous', sometimes referred to as 'single radio' or 'two radio', respectively. A 'selectable' Dual-Band device can operate in either 2.4GHz or 5GHz mode; a 'simultaneous' Dual-Band device can operate in both frequencies at the same time.
The 2.4GHz band is the same frequency used by a lot of other wireless devices, such as cordless phones and microwave ovens. Because there are more devices competing for space in this band resulting in interference and congestion, and it may affect how well the router performs both in speed and consistency. The 5GHz band provides better performance and coverage due to less interference.