Lol - I just used the term "muddy" in my reply to your reverb thread
It basically means that the individual elements are not being heard clearly - and the usual culprit is too many instruments at the same frequency - ie guitars, keyboards, vocals, the higher bits of the bass can all share the same frequencies
This is where EQ can help as you can carefully "carve out" some space for the important instruments
Eg I usually start with my vocal (the most important bit IMHO) to get a good sound
When I add guitars I will often make a cut in the guitar frequencies around the key vocal frequencies
Individually the guitar may not sound great but when mixed with the vocal it sounds great
If you PM me your email address I can send you some of my own rough guides and "cheat sheets" to get you started before you start looking at books (and I have some great recommendations for books but I think it would be better to grasp the basics before diving in to a book as most assume a fundamental understanding first - which is a little daft!)