The first thing to understand is what a constitution is. It is a written set of basic laws that form the basis for the government of a country. In the US, the Constitution was created in 1787 and is still in effect today.
Constitutions are harder to change than regular laws because they are supposed to be the most basic laws that set out how our governments will be organized and run. However, sometimes they need to be changed as times and attitudes change. For this reason, there is a process for amending, or changing, a constitution.
A constitutional amendment, then, is a change that is made to the basic law of a country (or a state or other entity that has a constitution). One example of this in the US was the 19th Amendment, ratified in 1920, which changed the Constitution to guarantee that women would have the right to vote.