It means that you would adjust your skimmer so that the skimmate that is collected is really thick and dark. The opposite would be to skim wet which would be to set your skimmer so that the skimmate is more of a light tea color. Different skimmers are adjusted in different ways. Some, such as the Aqua C Remoras are set by raising or lowering the collection cup. The higher the cup, the dryer the skimmate. Most in sump skimmers are set by either adjusting a gate valve or raising/lowering a tube which adjusts the water level in the riser tube of the skimmer. In this case, raising the water higher produces wetter skimmate and lowering it produces dryer skimmate.
Some skimmers also have air adjustments. These are a real pain IME and tend to need frequent tuning. The air lines also tend to get clogged easily. These include the Seaclone skimmers and the Berlin Turbo skimmers. If you do have this type of skimmer the key is to set it so that you get the maximum amount of air injection without greatly reducing flow. Unfortunately, these skimmers tend to have a certain "sweet spot" and any deviation from that tends to decrease efficiency making it difficult to adjust from wet to dry skimming.
