Simply speaking, dual band Wi-Fi describes the two spectrums a device like a Wi-Fi access point can access using the two radios installed in it. This allows the access point to provide connection through the 2.4 or 5 GHz frequency bands. One radio connects devices to 2.4, and the other radio connects devices to 5 GHz.
The 2.4 GHz band has 3 non-overlapping 20MHz bandwidth channels and one 40MHz channel. The 2.4 GHz band is ideal for sending moderate amounts of data over long distances.
The 5 GHz band has 25 non-overlapping 20MHz channels, 12 40MHz channels, six 80MHz channels, and two 160MHz channels. Wi-Fi 5 devices only use the 5 GHz band. Wi-Fi 6 devices are capable of using the 2.4 and 5 GHz bands. The 5 GHz band is capable of delivering and receiving exponentially higher amounts of data but over shorter distances than 2.4 GHz.