A manometer is a device which measures pressure by forcing a water column up a tube.
Imagine that you had a glass tube formed in the shape of a "U" and you filled it half way up with water. (This is a manometer). If both ends of the U are open to the atmosphere the water levels will be at the same level. If you connect one end to your CPAP machine, the pressure will blow the fluid and force it down (and up in the other arm of the U).
At some point the forces due to the pressure and the weight of the fluid will balance. The difference in height of the columns of fluid in the two arms is measured and is used as a measure of the pressure of the CPAP.
So a pressure of 11 cm H2O means that the fluid levels would be 11 cm (about 5in).