As mentioned before, units are often groups of warriors operating together. They fight in a loose formation with gaps between each model. This gives the individual troopers freedom to move quicker over difficult terrain and enables them to take advantage of cover.
When you are moving a unit the individual models in it can move by any amount up to their maximum Movement distance. Remember that units have to stick together otherwise individual troopers or vehicles become scattered as the squad loses its cohesion as a fighting unit.
Once a unit has finished moving, the models in it must form an imaginary chain where the distance between one model and the next is no more than 2".
During the course of a game it's possible a unit will get broken up, usually because it takes casualties or individual models are engaged in close combat. When this happens, the following rule applies:
If a unit is broken up for any other reason the models in it must be moved to restore coherency in the next movement phase. If they cannot do so then the unit may not shoot or launch an assault until it is in a coherent formation again.
Models engaged in close combat are not bound by the above rule. Once combat has finished the unit must form up again.
Diagram: Unit Coherency. Three example squad layouts illustrate the coherency rule. Two layouts are marked correct (check): models fanned out with 2" gaps between each neighbouring model in the unit, forming a continuous chain. One layout is marked incorrect (X): a squad of Space Marines split into two clusters with a 4" gap between them, captioned "This Squad has broken its coherency. The Space Marines on the right are more than 2" away from the Space Marines on the left."