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.