A taxpayer is somebody who's required to pay taxes to the federal, state, and/or local government. Businesses—even estates and trusts—can be taxpayers, too.
Lots of people share the same name. How does the IRS identify them?
An individual uses their social security number as identification. If they don't have one, they can get a taxpayer identification number. Businesses use their federal employer identification number.
What does a taxpayer need to do?
A taxpayer needs to stay aware of which tax returns they need to do, and which filing dates apply to them. They'll need to meet those deadlines to avoid penalties.