The Constitution of the United States is referred to as a "living document" because it the architects of the document intended for it to be adapted by future generations. It is because it is adaptable, that amendments could be ratified, or added to it.
The Constitution is also open to interpretation by the U.S. Supreme Court. When issues, such as abortion and voting rights, are presented before the court, the justices can use their interpretation of the Constitution to determine how to protect the rights of citizens and even how to restrict some rights. Changes to the Constitution are not easily made. An amendment starts as a proposal to Congress.
After debate and reviewing the ramifications of making a change to the U.S. Constitution, Congress votes on the proposal. It takes 2/3 of Congress to approve the proposal. It is sent to the states for a final vote next. If 3/4 of the states affirm the proposal, it becomes an amendment. Once an amendment is ratified, it can only be changed or repealed through another amendment. For instance, the 18th Amendment that prohibited the sale of alcoholic beverages was rendered ineffective by the 21st Amendment. However, because the Constitution is a living document, the Supreme Court can interpret amendments through different standards, including what is considered acceptable by society at the time.