Probably nothing, if your case has already been confirmed.
If you case has been confirmed (approved by the Court and chapter 13 trustee with a court order), then you are required to make monthly payments from 36-60 months, depending on your plan. If you miss one payment, the trustee will send you a letter. If you miss two payments, you’ll get a strongly worded letter threatening a motion to dismiss. And once you miss that third payment, you receive a motion to dismiss. Even then, your attorney may be able to object and move to allow you to catch up on those payments.
So, missing one payment usually isn’t the end of the world.
On the other hand, if you miss a payment before you case gets confirmed, it can hold up or even stop the confirmation process. You case can be thrown out with a motion to dismiss for that one payment. Prior to confirmation, you really need to make those payments. (Post confirmation too, but you get a little leeway).
Plus, your attorney gets paid out of those payments to the court each month, so he definitely wants you to keep up on your payments.