These are for safety and convenience, and are not seen since knickers are worn beneath the jacket. While knickers are not always worn in practice, especially if only fencing foil, knickers are required in tournaments. Since knickers only go to just below the knees, socks cover the rest of the leg. While not a must in practice, socks that are high enough to cover the entire exposed portion of the leg are required in tournaments. The feet can be cut off a pair of socks to make extensions if your legs are too long or your socks too short.
Socks are usually white, but can be other colors.
There are no special fencing socks, but finding tall enough socks is challenging enough that fencing retailers do stock them. Shoes Shoes used for fencing take a lot of odd abuse, and normal shoes tend to fall apart in well-known places after heavy usage. Shoes from other sports can be used quite successfully, but serious fencers often get special fencing shoes. Good shoes for fencing should be light, flexible, snug to the feet, and gym-floor friendly. Fencing shoes are built with this in mind, as well as tread designed for the strip, reinforcement for the stresses of fencing, and design that looks good with the entire uniform.