As you keep progressing within the game at around half way into the campaign Bomb Icons will start appearing around the map.
These type of missions requires you to defuse three bombs around a designated area without been spotted. Soldiers will be patrolling the area, and if you get spotted, you will get a 60 seconds counter to defuse any bombs you have left to do.
These missions are very easy as long as your're using an stealth approach. I recommend using your bow, or the sniper rifle suppressor for easy long range kills. And use your stealth take-downs for any unsuspecting soldiers left. After defusing all the bombs, just leave the area, and the mission will be completed.
Very easy, so have fun.