This is a form reporting cancellation of debt. Let’s say that you stopped paying on one of your credit cards. The company may decide the debt is uncollectible and write it off. They can then send you a form 1099-C.
The amount of this written-off debt must then be reported on your tax return. This cancelled debt is “imputed” to you as income.
You will pay income tax on the amount reported on the form 1099-C. For example, you have a 1099-C for $10,000. If you are in a 25% income tax bracket, then this imputed income will cost you an additional $2,500 in income tax.