Refers to the diameter in millimeters of the hoselines.
38mm = "inch and a half"
44mm (some use '45') = "inch and three quarters"
65mm = "two and a half" ("twin 65s" = two lines of 2 1/2 laid in parallel)
77mm = three inch
100mm = four inch
125mm = five inch
100 or 125 may be termed "high-vol" (for high-volume hose) - or LDH for large diameter hose.
In Redwood we started using these terms over 15 years ago... some departments are still using the imperial measurements.
Our handlines are 44mm (we did away with 38mm hose several years ago), our "large handlines" and standpipe etc hoses are 77mm, and our hydrant and supply lines are either 100 or 125 depending on which truck it is (some trucks use 100 because the hydrants are set up with fittings of that diameter, while the 125mm trucks are designed for rural operations where we'll draft from a body of water with suction hose).