A dual-band wireless radio contains two different radios. The first radio, the 2.4GHz radio, has short-range and can send out relatively few bits per second (Mbps). This makes it ideal for things like web browsing—you don't need to move much data around to get your webpage.
In a nutshell, Dual-Band Wi-Fi allows you to connect to two Wi-Fi networks at the same time.
Dual Wi-Fi frequency has dual-band for 2.4GHz and 5.0 GHz. Meaning, the phone can connect to either the 2.4 or 5 GHz networks. 
The main advantage is to solve the traffic jam in a crowded wireless network. You can connect to 2 separate routers at different frequencies and get double the speed where you need it most. For example, you can use fast 5 GHz for Gaming / HD video streaming / etc while connecting to a less congested 2.4 GHz band for normal daily tasks such as Web browsing, email, social media, etc.
Benefits of Dual-Band Wireless Networking

1. Reduce Interference

When you realize that your wireless performance is not what it used to be, the problem could be caused by other wireless devices. Usually, these are other people's Wi-Fi networks or cordless phones in the neighborhood. The best way to solve this problem is to change your router's frequency so that it can avoid interference with other signals.

2. More Network Capacity

A dual-band network gives you more bandwidth because two separate radios operate within each band, increasing the throughput of both bands compared to a single band network using one radio for both bands. For example, if you have 2 AC1200 links on 5GHz giving 300Mbps each then putting them together will give you 600Mpbs total speed compared to a single 300Mbps link. You can use the less congested 2.4GHz band for normal daily tasks such as Web browsing, email, social media, etc while connecting to 5GHz for HD video streaming/gaming / VoIP / etc where you need more speed.

3. Double the Distance

The higher the frequency of your router's wireless signal is, the shorter its range will be. The lower the frequency is, the longer its range will be. This is why dual-band routers can double their effective wireless coverage because they use one low frequency (2.4 GHz) for close-range communication and another high frequency (5 GHz) for long-range communication with your devices.