An array is an ordered list of values; the values inside it are called the elements of the array. Every array element has a number, an index, you can use to access it.
You can define the elements of an array between square brackets, like so:
For the following array examples, I'll be using strings (basically letters and words), though the array elements could be anything.
You can think of the dimension of an array as the number of indexes, or indices, you need to name a value of that array.
Think of dimension in geometry: 1d, 2d, 3d. We have the same thing with arrays.
A 1d array, or a 1 dimensional array, is the default array you think of when presented with an array.
Here is a simple example, of length 4, expressed in the form of an array literal (where all the elements of the array are shown between square brackets).
A 2D array is an array of arrays, where each array element contains its own array.
This 2d array is created using the Array() constructor, with several index values being filled with array literals. The array arr has 3 elements, and each element has its own array, of lengths 3, 5 and 2.
let arr=new Array();
You can access its elements by specifying the array index number you need. The example below would access the  element and print the character 'e' to the console.
If a 1d array is a regular array, and a 2d array is an array of arrays, a 3d array adds one more layer: it's an array of arrays of arrays.
let arr0=new Array();
let arr1=new Array();
This code would access the  array element and print the unique string 'team' to the console.