A pocket in Tomorrow's terminology is similar in function to a sub-account. You can visualize "pockets" as specific "pots" that store funds separately from your main account. Generally, these are used to assist you in budgeting (for example, having a "Pocket" for rent is convenient because you always know how much money you have aside from rent), saving for a specific goal (like a trip to Spain in a few months) or for monitoring cash flows.