During fermentation a thick layer of brown, gunky foam forms on top of the wort and sticks to the walls of the fermenter. This layer is known as the Krausen (from the German word "Kräus" which means "curly" or "frizzy"). This is why the high growth or attenuation phase is also sometimes also referred to as "high Krausen". Even if no bubbles can be seen to
escape through the airlock (e.g. due to a gas leak in the fermenter) the presence of Krausen is a sure sign that the yeast is active and that fermentation is taking place.
The Krausen mainly consists of living and dead yeast cells, excess proteins and other byproducts of yeast growth, as well as some compounds derived from the hops. It has a nasty and often intensely bitter taste, which is why the wort should never be stirred at this point to prevent these flavours from ending up in the beer. Eventually the Krausen will settle out and become part of the gloopy sediment that collects at the bottom of the fermenter when the fermentation winds down.
Krausen may be taken as an indicator of how vigorous the fermentation proceeds, because the amount of solids and semi-solid goop floats to the top of the fermenter is a function of the buoyancy these compounds receive as a result of released CO2. A slow fermentation produces a thinner layer of Krausen than a quick one does.
However, Krausen thickness is also a function of wort viscosity. If your wort has a higher viscosity and/or a composition that creates bubbles with a higher surface tension, the Krausen material has a higher capacity of retaining released CO2. This is why a "thin" wort can have a thinner Krausen layer than a thick, gloopy wort even if both worts ferment with the same vigor.
How much you should care is a matter of opinion, I suppose. There are several indicators of fermentation, including Krausen development, CO2 escaping from the airlock and gravity attenuation. If any of these indicate you have a good fermentation, I wouldn't worry about what may affect the others. At the end your hydrometer is the sure-fire way of measuring what goes on: fermentation is the conversion of sugar into alcohol which creates a corresponding gravity attenuation. So if your hydrometer tells you the gravity drops as per the expected attenuation, that's all you rally need to know as far as the progression of the fermentation is concerned.