You have filed your returns but did not 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.
Penalties and interest can add up fast. If you do not have the full amount to pay, do not ignore the balance due, it will only create a bigger problem. You have options available to you, including an installment agreement to pay the debt over time.