IRS means Internal Revenue Service
The IRS is the revenue service of the United States federal government. The government agency is a bureau of the Department of the Treasury, and is under the immediate direction of the Commissioner of Internal Revenue. The IRS is responsible for collecting taxes and administering the Internal Revenue Code, the federal statutory tax law of the U.S. Its duty to maximize tax revenue entails providing tax assistance to taxpayers, as well as pursuing and resolving instances of erroneous or fraudulent tax filings. The IRS has also overseen various benefits programs, and enforces portions of the Affordable Care Act (Obamacare).
In the 2015 fiscal year, the IRS processed almost 240 million returns and collected approximately $3.3 trillion in revenue, spending 35¢ for every $100 it collected.
Source: Wikipedia