I didn't know this (everyday is a school day), But here is the explanation for Hazlinnys 'Crossbar' if you require it.
Coastal bars are shallow, shifting sandbanks at the entrance to rivers and coastal estuaries. They can have strong currents and large breaking waves. You may need to cross a coastal bar when going out or coming in from open waters.
[and obviously]
It helps to keep net in position.