If you don't have enough money in a Jar you've opened with a merchant to complete a transaction, the balance will be taken from your Wallet. If there's not enough in your Wallet to make up the difference the transaction will be declined.
If that happens you'll need to transfer money from your bank to your Wallet (please note that transfers from your bank can take up to 2 hours to arrive in your account).
You can set a back-up payment source for your spending by clicking on three dots in the top right corner of the jar. For example, you may want your notonthehighstreet merchant jar to be backed up by your Christmas Jar