You have filed your returns but didn't have the money to pay what was owed. You may think, "Oh well—I will catch up next year." Before you know it you find yourself several years in arrears and suddenly there is a notice from the IRS, stating that you owe three or four times the original amount.
It's truly amazing how fast tax penalties and interest add up. Now you have a choice, you can write a big fat check and pay the full amount, including interest and penalties. Or you can just keep ignoring them while the penalties and interest keep piling up.