Full details of this limited time promotion can be found here: http://chipotle.com/chiptopia.
Here are the basic details of the program. Basically you are rewarded with a free entree for every 4th purchase totaling $6 or more and you earn "status" of either mild=4 purchases, medium=8 purchases, or hot =11 purchases which can lead to additional bonus awards if you maintain status for all 3 months. There are a few small details that make this program a little confusing. First, you must be 13 years old to participate which will likely apply to anyone reading this. Also, your free entrees for earning "status" will credit to your account within 24 hours and will expire after 30 days so make sure you don't forget to use them. However, these purchases will not count towards accruing higher status so keep that in mind. The only exception is that the mild and medium bonus rewards expire on December 31, 2016 and the hot status bonus reward (catering for 20) expires on March 31, 2017 so that gives you a little bit of time to redeem. Finally, your status resets the end of each month.