Orthographic depth is more or less comparative to the rate at which a child (or learner of the language) learns to read the language. English or French, for instance, which have absolutely no concept of following a phonemic pattern, have a greater depth than languages like Italian or Czech which tend to be read much more phonemically.
I can’t speak Czech (well, I can, but it’s not very good), but I lived there and I can pronounce every single word in their language without difficulty because there’s never any variation. The letter is there because it is meant to be pronounced and it is always pronounced the same way. If you give me a word in Czech, whether or not I know what it means, I can pronounce it. If you give me a word in, say, Arabic, I’m screwed because the language is dependent on the context of the other letters around it and the situation. Like English and French.
There is not a lot of data on this that isn’t in the academic journals of linguists, but if you get a chance and you’re interested (Ally!) it would behoove you to check it out.