Reinforcing nylon thread is usually used to reinforce the heels and toes of 100% wool knitted socks in order to reduced wear. The thread is held together with the wool yarn when knitting the heels and toes.
Wooly Nylon which is a stretchy thread used in sergers for sewing can be used as a reinforcing thread. This thread is available in a variety of colors at fabric stores and is relatively inexpensive.
Some sock yarn is sold with reinforcement yarn such as Lang's Jowell. This yarn is 75% superwash wool, 18% nylon & 7% acrylic which also helps with wear since it is not 100% wool.
Some knitters complain that using reinforcing nylon thread in knitted socks actually increases the wear of their socks and causing them to darn their socks more often. However, it is usually the wool yarn in the areas where the thread is used that wears. This does, however, give a good base for using duplicate stitch for darning worn toes and heels.
Another complaint about reinforcing nylon thread is that it may make the heel and toe areas of the sock thicker and stiffer. This can make the sock uncomfortable to wear.
Instead of using reinforcing nylon thread, some knitters use a smaller needle when knitting the heels and toes of their socks. This helps the durability of the knitted sock but doesn't usually affect the size.
Whether or not you knit socks with reinforcing nylon thread is ultimately a personal decision. Some knitters find it aids in reducing the wear of their knitted socks while others find that it is too much hassle and doesn't help with wear.