A constant is a container containing a value. This value cannot be changed apart from initialization, hence the name constant, as opposed to a variable of which the value can vary. A constant can be of any datatype and can contain a value according to its datatype. For example, an integer has whole numbers between -2^31 (-2147483648) and +2^31-1 (2147483647) (e.g. 23) , a float has a floating point number (e.g. 2.674) and a string has a series of characters (e.g. "Hello World!").
There is a number of predefined constants. These variables are predefined, which means they exist without the programmer making them. They can be useful in a some functions or when assigning them to some global variables.