Selective incorporation prevents states from making laws that infringe on the rights of U.S. citizens, as defined in the Bill of Rights. This is defined in the 14th Amendment to the U.S. Constitution.
The 14th Amendment was created after the Civil War to protect the rights of freed slaves. It addresses issues such as who can be a citizen of the United States, due process and the right to vote. The rule was first applied by the Supreme Court in 1873 regarding a monopoly in Louisiana, and later regarding issues around free speech, the right to legal counsel and education.