As stated, the tax system is pay-as-you-go. Each year, you must pay into the system over the year, either through withholding or estimated payments. If you owe more than $1000 when you file your return, you can be assessed a penalty for underpayment, even though you got caught up by April 15. (There are some other conditions for an underpayment penalty, but the $1000 rule is the easiest to explain.)
If you switched from a job to self-employment, you need to make quarterly estimated payments. Or, if you retired, you need to increase your withholding from social security or your pension or retirement funds.
If this is the first time you have owed a penalty and you received a bill, you can request a waiver or relief for cause.
https://www.irs.gov/payments/penalty-relief-due-to-first-time-abate-or-other-administrative-waiver
