It is said that TAX is included when people buy a ticket. What's TAX?

Taxes on international flight coupons consist of taxes levied by each government in the respective countries and airport taxes.
So the details vary by departure countries and destination countries.
For example of tickets for U.S. departure and arrival, in addition to airport taxes. there are US INTERNATIONAL TRANSPORTATION TAX,
Customs fee (US CUSTOMS FEE), US ANIMAL AND PLANT HEALTH INSPECTION SERVICE USER FEE, US IMMIGRATION USER FEE.