Variables are references that stand in for a value that is contained at a certain memory address.
Variables are said to have a value and may have a data type. If a variable has a type, then only values of this type may be assigned to it. Variables do not always have a type.
A value can have many values of many different types: integers (7), ratios (1/2), (approximations of) reals (10.234), complex numbers (4+2i), characters ('a'), strings ("hello"), and much more.
Different languages use different names for their types and may not include any of the above.
An assignment statement is used to set a variable to a new value.
An assignment statements in wikibook pseudocode is written as
let variable := value.
let X := 10
The example set the variable X to the integer value of 10. The assignment statement overwrites the contents of the variable and the previous value is lost.
In some languages, before a variable can be used, it will have to be declared, where the declaration specifies the type.
But this is not needed in wikibook pseudocode.
Variables store everything in your program. The purpose of any useful program is to modify variables.