“Data Stash” T-Mobile's plan for your unused data.

  The intention is that its customers might only use 3GB of your 4GB plans this month, so it would allow you to keep that extra unused 1GB and use it in the next month. This brings you to their terminology of stash, as you're stashing your unused data for the next month.

  T-mobile plans to start the new year with Data Stash. It will be automatically available to new and existing customers who are signed up for a “simple choice plan” of either 3GB or 5GB including tablet plans with 1GB or more. To kick-start Data Stash’s debate, they will be giving customers on these plans an extra free 10GB of data.

T-mobile has also stated that your Data Stash is good for up to a full year.

Keep the following in mind

 T-mobile mentioned within it's fine print that you will not start banking your unused data until their initial free 10GB is used up