To help you prepare for Modern Warfare's big update next week—which is apparently 'the biggest free content drop in Call of Duty history'—Infinity Ward has turned on double XP which will remain active until December 2, right before the launch of Season One.
If you've fallen behind in your progression or just want to get that extra boost before your current rank is 'locked and memorialized' on December 3, now is the perfect time to earn those extra levels. While the boost doesn't apply to missions or challenges, there's still a significant amount of potential XP to be gained just by playing. It's also worth noting that the boost doesn't stack with any XP tokens that you might have stashed away, so hang on to them for now.
Here's the blurb from Infinity Ward on exactly what happens once Season One gets underway:
"At the start of Season 1, those who have made it into the Officer Ranks (Rank 56+) will have their Officer Rank refreshed, beginning a new journey up the Season 1 Officer Progression track (from Rank 56 to 155). Players will find another 100 ribbons to earn through a new set of challenges, leading up to a new and distinct animated emblem."
If you're still confused, here's a handy blog post that explains Modern Warfare's player progression in more detail. And if you're looking for more info on the Season One content, which includes new maps, modes, and Special Ops content, check out Andy's post from earlier this week.