An important thing to understand is that tree rings come in two parts, earlywood and latewood. As might be intuitively obvious from their names, earlywood is composed of cells generated during the early part of the growing season while latewood is composed of cells formed during the late part of the growing season. In conifer species (e.g., pines, firs, and many others), the predominant cell type is called the tracheid, and earlywood tracheids are large but thin walled while latewood tracheids are smaller but thicker walled. This pattern intra-ring cell anatomy results in earlywood appearing typically light in color and latewood appearing dark in color, as is clear in the image above of four actual conifer rings growing from left to right. Thus, each real conifer ring has two distinct bands of color.
In the applet, rings are simulated as if they were from a conifer, i.e., each ring starts with a light earlywood (yellow) that is followed by a dark latewood (brown), as in the virtual example above. The color of latewood often varies in certain conifer species (e.g., spruces), as is illustrated in the image above of real rings with the 4th ring having a darker latewood than the other rings, and the applet accordingly has three different shades of latewood. Latewood color plays no other role in the applet for now, but many dendrochronologists crossdate their collections based on latewood color versus ring width in sites where ring width is complacent but latewood color (true wood density) is sensitive.
Note that in the applet the first ring is labeled with a "0" (the zeroth ring), and every 10th visible ring is marked and labeled. You may move the virtual core left and right by clicking and holding the left mouse button over the core and then moving the mouse. To see some of the very narrow rings, you may need to magnify the core series. To do this, move the area of interest to the center of the applet space (a blue line marked on the core mount) and click the core magnification setting to either 2x or 3x.
Occasionally, environmental conditions during a growing season are so extreme (e.g., a drought) that a tree can not apply a full cone of growth over all of itself. Perhaps the tree can maintain leaf, root, and cambial tissues during that year but not have enough additional photosynthetic production to apply any new wood cellulose. In this case, there would be no "annual" ring for that year, and this ring-growth anomaly is called a "missing" or "absent" ring. It is not likely that every tree in a particular forest stand will miss the ring for that year, and therefore presumably at least one of the dendrochronologically sampled trees in the stand will have at least a narrow ring for that year. By matching growth patterns across trees, absent rings can be discovered and accounted for, thus preserving the true chronology of the tree growth. In the applet, the default setting is not to allow for missing rings, but you may turn that option on and restart a new core to try a harder crossdating problem that may include any number of missing rings. If there is a missing ring, the virtual core will show nothing for it; you will have to crossdate your skeleton plot with the master chronology to discover the missing ring.
Conversely, environmental conditions during a growing season may be bimodal (e.g., spring moisture from snow melt, then a dry period, then summer rains) such that a tree applies two apparently full cones of growth over all itself. In this case, there would be two "annual" rings for that year, and this ring-growth anomaly is called a "false" or "intra-annual" ring. Although the intra-annual false band often differs anatomically from the true ring boundary enough so that it may be identified by it appearance along, this is not always the case. Again, it is not likely that every tree in a particular stand will have a false ring for that year, and therefore presumably at least one of the dendrochronologically sampled trees in the stand will have just one ring for that year. By matching growth patterns across trees, false rings can be discovered and accounted for, thus preserving the true chronology of the tree growth. In the applet, the default setting is not to allow for false rings, but you may turn on that option and restart a new core to try a harder crossdating problem that may include one false ring. If there is a false ring, the virtual core will show it as identical to its other intra-annual band; you will have to discover the false ring by crossdating your skeleton plot with the master chronology.
Note that even with these options turned on, your randomly generated virtual core may not have a missing or false ring. By clicking the "Hint" button, you can see if these growth anomalies actually exist in your current problem. If they exist, then the answers to the problem will the calendar dates of the missing rings and the ring numbers of the intra-annual bands, in addition to the sample start and end years.