Penal interest is a fee charged when you don't pay your EMI on time. It's usually calculated on an annual basis.
For example, if a lender charges a penal interest of 24% per annum, it means a 2% penalty is applied to the overdue amount for each month of delay.
Let's say your personal loan EMI is Rs. 50,000 and you miss the payment for a month. With a penal interest of 24% per annum, the late payment penalty would be about Rs. 1,000 (which is 2% of Rs. 50,000).