Band steering is functionality that automatically steers anyone connecting to a wireless network to the best frequency band available and supported, thereby optimizing performance for the client.
A frequency band is a section of a larger frequency area. Wifi uses two frequency bands: 2.4 GHz and 5 GHz.
If your equipment does not support band steering, it is up to the user or the device to select the correct/best frequency band. Many routers are set up so by the vendor or service provider so that one has separate networks with different names and credentials for 5 GHz and 2.4 GHz. It is then up to the user to know which network and thus which frequency band to connect to.
At worst, the user tries to connect to a network that is not supported by the device, or a network that provides much slower performance than the device actually supports.
See also Five reasons not to buy a Wi-Fi repeater.
Band steering means you don't have to worry about which frequency band is supported by the different computers and devices that you use, because you have a router or other wireless access point that can make these assessments and direct each device to the appropriate frequency band.
Band steering is often part of a more comprehensive functionality called client steering, which ensures that all clients (devices) connect to the wireless access point that provides the best performance.
Article by Geir Arne Rimala and Jorunn D. Newth