The incorporation doctrine is a constitutional doctrine through which parts of the first ten amendments of the United States Constitution (known as the Bill of Rights) are made applicable to the states through the Due Process clause of the Fourteenth Amendment. Incorporation applies both substantively and procedurally.
Prior to the doctrine's (and the Fourteenth Amendment's) existence, the Supreme Court found the Bill of Rights to only apply to the Federal government and to federal court cases. During the signing of the Constitution, every state in the negotiation had different levels of concerns with a too powerful Federal government, and the preamble to the Bill of Rights highlights the importance of the Bill of Rights in limiting overreach by the newly created government. The Supreme Court noted that the Bill of Rights was clearly intended to limit only the federal government (see Barron v City of Baltimore (1833)). States and state courts could choose to adopt similar laws, but were under no obligation to do so.
After the passage of the Fourteenth Amendment, the Supreme Court, through a string of cases, found that the Due Process clause of the Fourteenth amendment included applying parts of the Bill of Rights to States (referred to as incorporation). A lot of contention surrounds whether the Fourteenth Amendment should incorporate any substantive rights, with opinions from Supreme Court justices ranging from complete to no incorporation ( see substantive due process). Rather than find that the Due Process clause incorporates all of the Bill of Rights, the Supreme Court supported selectively incorporating rights that the Court finds as essential to due process. Under selective incorporation, the Supreme Court incorporated certain parts of certain amendments, rather than incorporating an entire amendment at once.
Some argue that the Privileges or Immunities Clause is a more appropriate textual basis than the due process clause for incorporation of the Bill of Rights but because Slaughter-House Cases dealing with this clause are surrounded by controversy this theory is not supported by the majority of the court.
As a note, the Ninth Amendment and the Tenth Amendment have not been incorporated, and it is unlikely that they ever will be. The text of the Tenth Amendment directly interacts with state law, and the Supreme Court rarely relies upon the Ninth Amendment when deciding cases.