Freezer bag cooking has become popular recently as an inexpensive way to prepare quick-to-cook meals for backpacking trips. Simply, meals are assembled at home in zip top freezer bags. Then, on the trail or at a remote campsite, the meals are prepared and eaten in the bags. This allows for inexpensive meals to be prepared that are light-weight to pack-in (because there is no water). Clean up is simple.
For the geezer Scouts reading this, you know that the recent freezer bag cooking fad is not new at all. Us old Scouters made omelets, ice cream, and fudge in freezer bags decades ago.
A couple of things to keep in mind. When you open the bag after it has been in boiling water or after you have added boiling water, you can get burned by the steam - so be careful. Also, don't go cheap with the brand of bag that you use. Even with the higher quality, thicker bags, you may need to "double-bag" if cooking in boiling water for an extended period of time.
I've gotten a few e-mails concerning the safety of cooking in freezer bags. Lucky for you, Scoutdude has a Ph.D. in chemistry and 25 patents. My verdict after reading peer-reviewed scientific journal articles (not watching Oprah) -- cooking in a freezer bag will not kill you, make you grow an extra eye, or cause you to go bald.