When you roll a skill check, if you roll a natural 1 or 20, this extension will make an additional roll on a table (from the CritsAndFumbles module) associated with that skill.  The result is displayed in the chat window.   This also applies to Concentration and Initiative checks.  Note you must enable the "Crits and Fumbles" extension before loading your campaign, and activate the module (same name) after opening your campaign.