An IRS agent contacts you and tells you that you owe back taxes and must pay immediately. If you don’t, they say they will have the police come and send you to jail. There are variations of this scam, such as local law enforcement contacting you with a warrant because you missed jury duty, but almost always you will be asked to pay immediately via wire transfer or even gift cards, and will be told to stay on the phone throughout the entire payment process (including driving to the store!). Or you may be asked for your personal information to confirm your innocence or to receive a tax refund. Do not fall for it! Most likely you are not communicating with a government agent.
If you think you really may owe on your taxes, go to the official IRS website to find a real IRS phone number to contact to confirm. You probably would have received letters from them first if you really owe money. Also, the IRS won’t immediately send you to jail – there would be a process you would be aware of prior to any criminal punishment