The item drop system is the process of item distribution within Team Fortress 2. It distributes a random item to players on a regular basis until a weekly time-cap is reached.
This system was first introduced with the Sniper vs. Spy Update to change distribution from being dependent upon achievements. Its purpose was to improve accessibility to the expanding quantity of items within the game, while negating the need to achievement farm (meaning to perform out of context actions to gain achievements) or use third-party tools to falsify the obtaining of achievements. However, this system created another way of farming to obtain new items called idling.
Right now, the item finding rate is about 30-70 minutes for one item. In the June 10, 2013 patch, the system was changed so that you had to accept the item before moving on, to prevent other ways of idleing. The max gameplay that will be counted for item drops is about 20 hours a week, the rest will be carried into the next week. The reset time for counted gameplay is every Thursday.