Default means you have failed to pay as you’re supposed to. If it hasn’t already, the debt collection company will usually go through the courts to recover the money owed by you.
If it’s over six years (I think it’s six, could be seven) since you acknowledged the debt, then they may be too late to go to court over it, but may continue to contact you in an effort to get you to pay.
If it has already gone through the courts, then you will have to pay it, and until you do, interest will be increasing the total owed all the time.

It may be worth going back to the origin of your debt. I did get into debt quite along time ago and received letters from debt collecting agencies saying I owed ‘ x amount ‘ and additions that the debt collectors had added on. I ignored these letters and went back to the original source. They agreed to accept payment for only the amount I owed and therefore saved me debt collectors additions. Default, as I understand it is a term that will be added to your credit record and will show when future potential lenders credit check you. It shows you have not adhered to the agreed repayment terms. It is not as bad as CCG and will be removed at some point in the future when you have repaid the debt.