Active defence is a rule variance for handling results of monster attack rolls. It is (to my knowledge) originally a homebrew rule from 'Darker Dungeons', and is meant to make defending have the same amount of dynamism and engagement as the other standard rolls.
When a monster attacks you, roll a d20 and add your AC as a dice check against the monsters attack. The monster DC is 22+ it’s attack bonus. On a natural 1 you are critically hit, on a natural 20 you parry the monster and may use your reaction to make an opportunity attack. If a monster has advantage against you, then you make your defensive roll with disadvantage. If a monster has disadvantage against you, then you make your defensive roll with advantage.
This module is designed to work with the DnD5E system. It is unlikely to work with any other system.
Within the FoundryVTT Setup screen, navigate to "Add-on Modules" then click on "Install Module". Within the "Manifest Url" field, write: https://raw.githubusercontent.com/Lazrius/FoundryVTT-Module-Active-Defence/master/Source/module.json, then click install.
To make an active defence roll, simply navigate to the desired actor's character sheet, and then click on the armour class attribute. This will open a dialog box which looks like the standard roll interface, with the added button "Generate Macro". Pressing this will create a macro that can skip this process, so you can simply click on a token and then run the macro instead.