Most threads are either two or three strands twisted together to make the final product. If a particular thread is labeled 50/2 or 50/3, the number following the slash indicates the number of plies or strands twisted together to make the final product. Each of those individual strands or plies also has multiple strands twisted together. One important component of thread quality is the number of twists applied to the thread. A loosely twisted thread will look bulky and fluffy on the spool. It will unravel and shred more easily than a tightly twisted thread. Imagine the thread moving along the thread path, through the tension discs, and through the needle. If it has a loose twist to it, it will easily be caught, pinched, or snagged. Slight snags will result in lint buildup as pieces are torn away from the thread. A strong snag or pinch will result in thread breakage. A tightly twisted thread has a smoother surface and will not get caught or snagged as easily.
Although it is never printed on labels, thread twist is measured by the number of twists applied per meter (approx. 3 feet). A loosely twisted thread requires less total fiber content, takes less time to produce, and is less expensive. It may have as few as 150 twists per meter. (Think of a budget serger thread that can easily be untwisted by rubbing it between your fingers.) A quality thread will have as many as 1,200 twists per meter, resulting in a smooth, consistent surface. A higher twist also condenses more thread into the space resulting in greater strength.
Here's an exaggerated example of how proper twisting affects the quality of thread: Take a large bath towel, lay it on the floor, and measure the length. Let's say it is 48 inches long. Roll the towel length-wise so you now have a rolled towel that is still 48 inches long. Start twisting the towel. Every 3-4 twists, re-measure the length and you will notice that you lose about two inches. Continue to twist the towel another 10 turns and the towel will be only about 36 inches long. We lost 25% of the length of the towel. The result, however, is a very smoothly rolled towel. The tightly twisted towel is also much stronger than a loosely twisted towel. The more twists applied, the smoother the surface becomes.
If we start with 10,000 yds. of untwisted thread and apply a loose twist, we will end up with 9,500 yds. of thread that is not high quality. If we apply the proper twist, the final measurement will be about 7,500 yds. A quality thread requires about 20% more fiber than a low quality thread. Like most things, you get what you pay for.