padave,
Many fly fishermen (in their minds) categorize trout streams by the difficulty level or, broadly speaking, the level of challenge required to catch trout. This is purely informal - there's no quantifiable aspect to this and as you'd guess FFers have different opinions on which streams are "harder" than others. Sometimes FFers refer to difficult trout streams as "technical" streams. Most streams associated with this type of reputation are well known, heavily fished, and have special regulations in the popular areas. Many if not most of the streams you'll see in the Reports and Streams section of this website are streams I'd categorize as difficult or technical waters - streams like Valley Creek, Little Lehigh, Yellow Breeches, Big Spring, or Penns Creek.
That's not to say that a beginner shouldn't try to fish them, just that they should understand that catching trout in these streams is tough even for those of us with decades of experience. While Codorus Creek isn't one of these famous legacy streams I find it difficult due to the physical structure of the stream. It is mostly wooded with lots of low hanging trees making casting hard and the slower, deeper pools have a muck bottom making wading almost impossible so you have to stay on the bank and cast between trees on much of Codorus. Also, wild brown trout are moodier, easier to spook, and more selective about what flies they want than stocked trout. By all means visit Codorus but don't expect if you're a beginner to catch dozens of trout.