Unit of pressure in the SI (systeme internationale) metric system.
Standard atmospheric pressure is "one thousand and ten millibars", i.e. 1.01 bar.
As you dive underwater, due to the weight of the water above you; the pressure increases by 1 bar for every 9.81 meters of depth.
To a good approximation, we say that the pressure increases by 1 bar for every 10m underwater.