Taxing power refers to the ability of a government to impose and collect taxes.
General Constitutional Authorization
In the United States, Article I, Section 8 of the Constitution gives Congress the power to "lay and collect taxes, duties, imposts and excises, to pay the debts and provide for the common defense and general welfare of the United States. This is also referred to as the "Taxing and Spending Clause."
Under the Sixteenth Amendment, Congress has the power to collect income taxes.
The Internal Revenue Code is the main law governing income taxes. The Internal Revenue Code is codified as Title 26 of the United States Code.
States are also allowed to impose and collect their own taxes, which is included but not limited to income taxes, sales taxes, and property taxes.
For more on the taxing power, see this University of Virginia Law Review article, this Arizona State Law Journal article, and this Berkeley Law Review article.