Bandwidth is the size of a wireless frequency channel. Think of it like how many lanes there are on a highway, or the size of a water hose.
The larger the bandwidth allotted to a channel, the more information that channel can carry.
The available bandwidth is what limits capacity on a wireless network - whether it's cellular or Wi-Fi. It's why when there are a lot of people sharing that same wireless signal, speeds become slower than when there's less congestion.