Plain English Meaning
The aircraft's own electrical and metal parts pull the compass needle slightly off, so the compass doesn't always show true magnetic north exactly. A small card near the compass tells the pilot how much to adjust for it on different headings.
Full Definition
The error in a magnetic compass reading caused by magnetic fields produced within the aircraft itself, such as those from electrical wiring, radios, engine components, and ferrous metal structure. Deviation is unique to each individual aircraft and varies with the heading flown. It is recorded on a compass correction card mounted near the compass, showing the heading to steer to achieve a desired magnetic heading.