A unit can move straight forward any distance up to its Movement value (M) in inches.
In Warhammer, a basic move for any model, and therefore any unit of models, is to move forward a number of inches up to their Movement characteristic.
Remember that individual models are not permitted to leave their units and so a unit effectively moves as a single entity. If, for whatever reason, there is more than one Move characteristic in the unit, then the entire unit is treated as having the same Move value as the slowest model.
Units are cumbersome, and find it hard to change direction. Models (and units) must move forward in a straight line, unless performing a maneuver, which we'll discuss in a moment.