I see that in some laptop specs.
Like in intel dual band wireless-ac 7260
What's the dual band good for?
There are various wireless "bands", which are particular frequencies. Each band has a number of channels, which are frequencies close to each other. When waves from the same frequency are in close proximity, they cause interference, which reduces overall bandwidth as data has to been re-transmitted, and can even cause a total loss of connectivity. Wireless-G supports only the 2.4GHz, as do many cordless phones, etc. Wireless-G networks cooperate with each other for bandwidth, but it reduces overall bandwidth for all devices, and interferes with devices that don't cooperate with each other.
Wireless-N and Wireless-AC supports both 2.4GHz and 5.0GHz, which means that if the 2.4GHz band is "crowded", you can use the higher frequency 5.0GHz, which is only used by new dual-band routers and some cell-phones. This can often greatly increase bandwidth for both older devices as well as newer devices. Cheaper Wireless-N routers can't use dual-band, so paying for a higher-end dual-band router can improve performance for the entire network and all networks in the nearby area (e.g. in an apartment complex or condominium). You must have both a dual-band network card and wireless access point to use the dual-band feature.
The 5.0GHz band also has a smaller wavelength, and so can carry more data a further distance than 2.4GHz networks (some claim by up to 300% more distance and a higher bandwidth capacity as well). Wireless-AC, the latest in wireless networking, has far superior performance compared to Wireless-N.