A constitution is the basic law that a country (or a piece of a country like a state) must follow. It is the law that establishes how the country's government will be organized and what things it will and will not be able to do.
A constitution is generally not a very detailed thing. It is much more basic. The Constitution of the United States, for example, is quite short. The reason for this is that it does not make the laws that we live by day to day. It does not set the speed limits or say what age people can be before they can get a driver's license. Instead, it says more basic things. It tells us that our laws will be made by a Congress. It tells us that we will have a president. It says that the government has to respect our freedom of speech.
Let us use the analogy of a family. The family's "constitution" simply says something like "the parents are in charge and the kids have to obey, but the parents must try to do what is best for the kids at all times." This is the basic set up of the family. But it does not go into detail by setting bed time or saying how old you have to be before you can go on dates or anything like that. The constitution is the basic law that says how the government will be set up.