The before modifiers means that a unit rolls two dice when attacking into or out of a forest and then you apply any effects that change the number of dice rolled. For example the Blood Harvester's "rage" ability. Or a melee unit attacking a flying unit. These effects modify the number of dice rolled, but they are applied after the forest's effect of bringing the unit down to 2 dice.
Let's say there is a Blood Harvester unit with two figures remaining. Normally due to rage this unit would roll a total of 4 dice in combat (3 base roll, +1 modifier due to rage). When attaching into or out of a forest they roll 3 total (base roll goes down to 2 due to forest effect, then apply +1 modifier from rage).