Wondering about thread count isn't likely to keep you awake at night (at least I certainly hope not), but if you occasionally ponder what it means (most likely when buying bedlinen) and realise you have absolutely no idea then read on.
Thread count is just one of a number of factors that determine the overall "quality" of cotton. It refers to the number of threads per square inch of fabric comprising the vertical (warp) threads and horizontal (weft) threads woven together, for example 100 vertical threads plus 100 horizontal threads would give a thread count of 200. Standard cotton wll be around 150 thread count, good quality would usually be considered to be 180 or higher, 200 or higher is often termed percale. Extra threads can be woven in, however, raising the thread count but not necessarily the quality (a little bit of manipulation) and beware very high thread counts with very cheap price tags (too good to be true in all likelihood).
Other factors in the quality of cotton are the country of origin of the cotton (with Egyptian cotton usually considered the best), the length of the fibres (with longer fibres creating stronger and finer yarns and therefore generally considered of better quality), the yarn size (the higher the yarn size the finer the yarn, with finer yarns giving more supple, lighter fabric and allowing more yarns to be used per square inch (there you go, back to thread count)) and the processes used in the finishing.
In a nutshell, therefore, thread count is exactly what it says (a count of threads) but in terms of determining quality is one factor and one factor only.