If one of the authors of a gem you use introduce a new version of the gem, there is a possibility the new version introduces backwards incompatible changes that would break your code.
Freezing a gem puts it into the vendor folder of your application, and it will not be automatically updated on its own. Rails will use this version of the gem instead.
This allows you to update the gem on your system for other apps, while having your sole app use the version of the gem that you have always been working with, and therefore is stable.
This applies to the rails gem itself as well, as newer versions of rails might eventually cause something in your app to break, freezing it will prevent the system wide update (and, once again, allows you to update other apps on your machine, while leaving the app you freeze rails in at that version number.
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