What are we actually celebration in the 4th of july?

The 4th of July is referred to "Independence Day." 
We celebrate the 4th of July for the remembrance of the acceptance of the Declaration of Independence on July 4, 1776. That Declaration of Independence stated that the thirteen American colonies were no longer apart of the British Empire, and formed into the United States of America. The thirteen stripes on the American flag stand for the 13 American colonies who declared their independence. 

The 4th of July, it is the birth of the United States of America.