This is a letter from your creditor warning that your account is about to default because you're behind with your payments.
The default notice will give you at least two weeks to catch up with any missed payments. If you can do this your account will carry on as normal. If you can't pay the missed payments in this time your account will default.
A default notice is normally sent when you've missed, or paid less than the full amount for 3 to 6 months.
Default notices only apply to debts which are regulated by the Consumer Credit Act, such as credit and store cards, payday loans, personal loans and hire purchase.