These markers are found on state and county highways, but not on the Interstate or US roads. Look for them in at the side of the road, sometimes at the end of a guard rail.
The letters indicate the county and the numbers indicate the mile it marks.
The marker in the picture is in Monterey County (MON) on CA Hwy 1 (1) at mile 58.
Unlike mileage markers in some other states, California markers are numbered in the same sequence going in both directions. If you were to look at the marker just opposite this one on the other side of the road it would be marked the same. The markers are not at regular intervals and are sometimes extremely close together (or far apart) for no apparent reason.
