Karma is the act of reliving lessons to gain resolution. Karmic debts are lessons that were failed in previous lifetimes.
When you are in a situation and your gut (True Self) tells you not to do something, but you do it anyway you can feel guilty. In feeling guilty, you judge yourself for your action and feel the need to pay for it. This then creates karma.
Once you have karma, you subconsciously summon karmic lessons in an effort to help you resolve your guilt. These are situations where you, or someone you love, experiences the same circumstances (sometimes in different forms). If you fail these lessons, you carry forward karmic debt that must be resolved in either your current or future lifetimes.
The energy of karmic debts along with guilt can grow stronger with time. You can think of karma as a debt collection agency, karmic debts as the amount owed, and lifetimes as interest on the original debt.