Pressure error is the difference between sensed air pressure (static or pitot) and freestream pressure (static or total) . This occurs because the local airflow affects the pressure around the port. In relation to an altimeter it means that the sensed static pressure will give an incorrect altitude reading unless it is corrected. Air Data Computers have correction tables based on flight test data which apply a correction to the sensed pressure to give a corrected pressure, from which the indicated altitude is derived.
On some aircraft if you select alternate static source in flight you get an uncorrected pressure measurement (or perhaps a less sophisticated correction) which means the indicated altitude jumps to a new value.
The glass isn't half empty, or half full, it's twice as big as it needs to be.