Countermeasures are special challenges which must be selected and activated when you are at a Control point (they will appear as a menu option after interacting with them).
Each of the available challenges is randomly generated (each with their own reward), and is usually related to killing Hiss in a specific way, in a specific area of the Oldest House.
You can only select/track 3 countermeasures at a time, so you won't be able to grab 5 straight away. Instead you'll have to grab a few, complete them, and then return to a Control point to grab some more. Simply complete 5 countermeasures in total to unlock this achievement.