Magnetic deviation is expressed in degrees of angle. It represents the correction that must be applied to a true course in order to get a magnetic course. As an example, in aviation, some information is given in true directions - generally, weather and wind information is handled this way. When you get told by a weather service that the wind is 230° at 6 knots, that means the wind is coming from 230 true - the angle between geographic north and the wind source. However, much of aviation navigation is done in magnetic, or in relation to magnetic north, because airplanes until recently generally only used magnetic compasses to determine their direction of flight. So to convert from true course to magnetic course, you need to know the local magnetic deviation.
Imagine standing outside. Now imagine pointing one arm directly at the North Pole. That's north, a familiar concept - the top of the map, and so forth. Now, however, point your other arm at the north magnetic pole. That's where a compass needle points. The angle between your two arms is the magnetic deviation. As it's expressed, if the deviation is to the east you subtract the deviation from the true course to determine the magnetic course you want, and if it is west you add it. The mnemonic for this is "East is least, west is best." Least means subtract, best means add.
In aviation, magnetic deviation for a location can be found (in the U.S.) either in the Airport/Facilities Directory, or on a VFR sectional chart which contains 'deviation lines'.