Lasalle represents each battalions ability to put up screen of skirmishers by giving each infantry unit a SK value. This value ranges from 0 to 3 and is shown by placing marker in front of the unit.
Skirmishing is a strange topic in Napoleonic circles and there has been may discussion on it's historical benefits and how to represent it on the tabletop. I personally like how lasalle treats skirmishers as I hate having to keep track of how many men are assigned to the task. It also removes the how effective were each nations skirmishers? British light company vs French voltigeurs? What about the same number of Prussian rifle armed jaegers? Assigning an abstract value works well as it creates an effect and not rely on a mini battle to produce that effect.
Skirmish factor comes into play in two situations during the game.
Firstly it provides your infantry unit an extra dice if your SK value is higher than the opposing infantry unit. Also if you position yourself well and can get to the flank or rear of an infantry unit you get the full SK value in bonus dice.
This may not seem like much but during the game and across multiple units this can jump you from 2-4 hits (1 disruption) to 5 hits (2 disruption).
Secondly the SK value is used to dilute the effects of canister fire. You take the SK value off the dice your opponent rolls for his canister fire. High SK value is important to reduce nasty canister shots from a 2 disruption shot to potentionally a single disruption shot.
There are situations where infantry lose their SK value. When charged or is charging or when in square formation you lose your SK value.
Certain nations can increase their SK values of their troops. The Prussian and British lists have SK bases in most of their core lists which you can add to your infantry units. These represent the rifle equipped battalions which could be assigned as needed. You can also break up an experienced SK2 or better unit to SK bases equal to it's bases (normally 4). This is almost always a bad idea as that unit could do more than an increase in SK value would.
A final note you only get bonus dice against other infantry units. Cav and cannons have no value so you don't get bonus dice but your canister fire is not reduced either.