Single band routers will typically offer just 2.4GHz for the greatest flexibility, as nearly all 802.11n wireless devices -- like tablets and smartphones -- will use that frequency.
The network bandwidth is handled separately, so if one band (like the 2.4GHz) is currently bogged down, you can connect to the 5GHz network for increased speed and throughput.
The 2.4GHz band is typically more effective in getting through walls, but the 5GHz band has an easier time offering faster speeds and theoretically broader coverage.
The RT-AC66U can set up three wireless networks over the 2.4GHz band and three more over 5GHz, giving added flexibility for guest network access.
Current Wi-Fi technologies typically operate on two main wireless frequency ranges: 2.4GHz and 5GHz.