The term 'move' refers to the physical act of moving your models across the play area.
Usually, the maximum distance that you may move any individual model in any single move is defined by the Speed value of the unit that the model represents.
Sometimes, the distance you may move units your detachment may be defined in a different way (as with Overwatch orders or a Broken detachment’s Retreat move).
Or, you may be allowed to chain more than one move together at one time — as with a triple move under March orders. Note that these are technically separate moves, so don’t just multiply the Speed value — in some cases, terrain may affect the Speed value of your units for one move but not for other moves in the same chain.
For each move that a detachment is allowed to make, you may move each unit in that detachment any distance up to the unit’s individual Speed value. Where you are allowed to make multiple moves, complete all moves with a unit before you move the next unit — don’t slow things down by moving an entire detachment on March orders three times!
[TODO: Improve text and add example(s)]