Earning Tickets in Modern Warfare allows you to take on Trials missions in the game. The Trials Missions are various missions that allow you to earn Star Ratings for the Trial and earn XP Boosts depending on how well you do. You can only do trials if you have tickets and this guide will explain what you need to do to get Tickets for the Trials missions.
To get tickets to use in trials you need to make it to the Officer Ranks. (This is past level 55) Once you are in the Officer Ranks you will earn tickets as you progress every certain amount of levels.
Each ticket that you have will give you three attempts at a Trial Mission. During this mission you have different criteria required for getting a star rating. If you complete the three-star rating requirement for a Trials Mission you will get the max XP bonus for this mission.
During each season in Modern Warfare, new trials missions will be added which allow you to play these matches and further progress through the Officer Ranks.