As a living being, in any of your births, you always receive favors or service from others souls. Sometimes you do not pay back to these favors and hence fall in debt. This debt is not necessarily financial or material debt, it is a debt about doing or servicing or giving to someone. Until this debt is cleared, your accounts are not even, and you are born again and again to settle your accounts.
When time will come to pay back your debt, God will give you some suffering as an indication. Depending on your birth chart, an astrologer can identify if you have a debt.
You are given a chance to pay back the debt by being born as a human being. One way to clear debts is by giving service without any expectation of return or any other motive. Such a service could be anything righteous and beneficial for the society. For example, you could contribute in any social, religious or humanitarian organization in the best of your capacity. Or you could do things like :service to old people, take care of little girls, take care of orphans, cook food for needy and distribute to them etc., the choices can be many. It should be a service that involves you personally and takes the effort out of you.
Serving someone should not come to you as a pride, or reason for expectation of return. You need to see it rather as a chance given by God, to pay back your debts.
By doing service, you receive blessing from others, and these blessings contribute to clearing your debts. By doing service, you complete yourself as a human being, as you evoke in yourself emotions like compassion and empathy. By doing service you become humble and wise.
It is advisable to find some time in your busy life to do service, without expectation, even if you are not in debt.