Airport operations personnel use friction meters or MU meters to measure braking action on the airport surfaces. It quantifies the braking action as a number which is then used by the airlines and frequently referenced by controllers.
This is from 3-3-4 in the 7110.65:
d. Furnish runway friction measurement readings/values as received from airport management to aircraft as follows:
1. Furnish information as received from the airport management to pilots on the ATIS at locations where friction measuring devices, such as
MU-Meter, Saab Friction Tester (SFT), and Skiddometer are in use only when the MU values are 40 or less. Use the runway followed by the MU number for each of the three runway segments, time of report, and a word describing the cause of the runway friction problem. Do not issue MU values when all three segments of the runway have values reported greater than 40.
EXAMPLE-"Runway two seven, MU forty-two, forty-one, twenty-eight at one zero one eight Zulu, ice."
