When they are saying "reading the water", it is in reference to seeing where the current/flow of the water is strongest and/or slowest and seeing the "feeding lane". I refer to the feeding lane as where flys, ants, and other various insects are being carried down the stream. Often on th eedge of the strongest flow of water through a run, the current slacks off and insects, flies, etc. are kicked out of the strong stream flow. Trout often "hold" is slower/deeper water in order to conserve energy and have cover respectively. So where the trout are holding and where there is food is critical in determining where to cast. When you are reading the water you are looking for where fish might be holding, stream flow, where insects are being carried down stream, and the insects in the stream; all this so you can know where and how to cast your fly (& what type of fly)! This is only some of things. Hope this helps. The best way to understand is to go fishing with an experienced FF.