When you are at an airport and you set your altimeter to the setting received from AWOS, ATC, ATIS etc. your altimeter will read your current "true" altitude above mean sea level (msl).
Generally what your altimeter is showing is assumed to be the airport's "airport elevation." The altitude displayed is usually very close to the published airport elevation. However, the "published" airport elevation is actually measured from the highest point on any usable runway, which may not be where you are actually located on the airport at the time you set your altimeter.
At Atlanta (ATL) for example, the published "airport elevation" is 1026 ft. above msl. However, the elevation at the approach end of Runway 27R is 985 ft. above msl, a difference of 41 ft. So, if you have ATL's current altimeter setting set and you are on the approach end of Runway 27R your altimeter should be showing 985 ft., (not 1026 ft.) which would be accurate.
@JohnK provides an excellent answer. But I thought it would be helpful to add some additional information to help you fully understand what is happening when you set the local altimeter setting while on an airport.