There are many ways to send money to a friend using your phone. PayPal, Venmo, Facebook, Google. Not long ago, Square introduced Cash as its own personal method for you to take money that's in your bank account and stick it in another's. Now the company is introducing a feature that lets you set aside money specifically for this purpose.
It's called the Cash Drawer, a place for you to save money you receive from other users or store cash that you've added from your own bank account. Two users with Cash Drawer enabled can send money from drawer to drawer, rather than going directly from one bank account (or debit/credit card) to another.
This is similar to a PayPal account. And like that older payments service, Square doesn't stop you from transferring money from your drawer to your bank account. Except when you tap Square's Cash Out button, the bank deposit should be instant. Cash Drawer is also opt-in, so you can proceed to ignore the addition if you don't see a point.
While your money is in the Cash Drawer, it's in Square's hands. That probably won't be much of an issue for people passing cab fare back and forth, but it is something to keep in mind.