That refers to an altimeter adjustment to compensate for local atmospheric variations in pressure. The reading given is what the atmospheric pressure would be on the field if it were at sea level in inches of Mercury (inHg). This is most often used in adjusting a sensitive barometric altimeter in an aircraft so that it reads an altitude which is within +/- 75ft of the actual value. These sensitive barometric altimeters are required equipment on IFR flights and much more useful on VFR flights as well.
ATC often give an altimeter readout when handing aircraft over to another ARTCC or approach controller, generally as a courtesy to the pilot and, in the case of IFR flights, to ensure the flight crew is using the correct altimeter settings. Other cases include at controllers at towered airports, in between the hourly ATIS broadcast update, when there has been a noticeable and fast change in the local barometric pressure at the field.
Off field and bush flying operations can make use of the same information in the form of portable weather stations such as Kestrel's series of handheld weather stations which provide barometric pressure readouts. Pilots also will just set their altimeters to the established field elevation listed on charts in lieu of an altimeter readout as well. The altimeter should then read to within +/-75 ft of the correct altitude, provided the unit was serviced and inspected within 24 calendar months of the flight.