This tutorial will give you an oversight of what arrays are.
oncrete commented example of use of arrays - LINK
From the how do I FAQ section "Arrays" that you should really check each time you make a search since it contains lots of answers to the frequently asked questions like those, the capx in the topic will show you how to properly use an array, hopefuly in a comprehensive way.
Also be sure to check the manual article about arrays.
An array is a structure in memory that allows you to keep values.
Contrary to a global or local variable which can hold only one value of one defined type at a time (number or text), an array is composed of cells, each one of which can hold values of wether type.
Those cells can also contain a dimension of the array.
Width is the X size/index, Y the Height and Z the depth.
Those are the coordinates of the cell in a 1D (only X), 2D (X, Y) or 3D (X, Y, Z) array.
This means that a cell from index X = 0 can contain more than one cell at Y and Z. (Check the tutorial again as well as high school mathematical notions)
An array makes it also easier to access/manage/loop through the cells
Hope this helps you figuring out a bit more the use of arrays.