You read that there were two entrenchments in the U.S. Constitution. How do you know that they exist?
An entrenchment is the placement of something in a trench (according to Merriam-Webster's Dictionary). This connotes a defensible and protected cavity (e.g., for warfare). Article V of the Constitution explains the entrenchments. Article V describes amending the Constitution, and it specifies two restrictions on amending the Constitution. One of them, on the importation of slaves until 1808, no longer applies (as it was temporal). The second entrenchment is the guarantee of states having equal representation in the Senate.
Here is Article V in its entirety:
The Congress, whenever two thirds of both Houses shall deem it necessary, shall propose Amendments to this Constitution, or, on the Application of the Legislatures of two thirds of the several States, shall call a Convention for proposing Amendments, which, in either Case, shall be valid to all Intents and Purposes, as Part of this Constitution, when ratified by the Legislatures of three fourths of the several States, or by Conventions in three fourths thereof, as the one or the other Mode of Ratification may be proposed by the Congress; Provided that no Amendment which may be made prior to the Year One thousand eight hundred and eight shall in any Manner affect the first and fourth Clauses in the Ninth Section of the first Article; and that no State, without its Consent, shall be deprived of its equal Suffrage in the Senate.Taken from the U.S. Constitution