As the phrase suggests, retirement income is money that you collect in retirement to pay for things like groceries and housing expenses.
The main difference between retirement income and regular income is that retirement income doesn't come from a full-time job. It comes from other sources --- like savings accounts, retirement accounts, pensions, and Social Security distributions.
Unfortunately, some people reach retirement age without anything put away in the bank --- and this is very dangerous.
As such, it's critical to plan ahead for retirement. That way, you won't get stuck in a situation where you have to keep working in your golden years.