I find the best way to think of Arrays is as an (Excel) spreadsheet - you have your columns and your rows, and each square (or 'Cell') contains a piece of data you want.
So in Row 1 in Excel I might have the scores of each level, with each level being in a different 'Cell' (Level 1 in Column A, Level 2 in B etc.).
The Array's Width basically coressponds to how many, in this case, levels that I want to save. So if I have 10 levels in my game for which I want to save the scores then I would set the Width of the Array to 10. The Height in this case I could leave as 1 as I only want to save scores.
If I wanted to also save, for instance, the time it took for a person to do my level then I would save that in Row 2, with the same Columns (A, B etc.) for each level as before. I would have to make the Height of my Array '2' to do this though as I now have two rows of data.
In Arrays though letters aren't used, and are also numbers. So if I want to access/store Level 2's Score, then I would want Cell B1, which in terms of the Array would be X = 2, Y = 1 (two columns across on the first row).
I realise this analogy is slightly useless if you've never used Excel, but it's how I tend to visualise it =)
I'm also going to upload a .capx at some point showing how to save/load Arrays to Webstorage at some point as I haven't seen a tutorial on this that doesn't used plugins.
Hope this helps!