Khuffs are socks that are made out of leather. They are a kind of moccasin. Often they are fastened with a zipper or a snap, or even both.

Khuffs are a traditional Islamic garment. The Prophet Muhammad, peace be upon him, liked to wear such socks both home and outside. He often wore them in prayer as well, and many Muslims today wear them in imitation of his example (Sunnah). It's not hard to find a pair in a Muslim marketplace.

Aside from being comfortable, the khuffs also allow an easier method of ablution (wudu) before praying. Normally, after washing the hands and face and arms and head, one is supposed to remove their shoes and socks and wash both feet with water (According to mainstream Sunni fiqh). If a person had put on the khuffs when they were in a state of ablution, and is wearing the same khuffs when it breaks, then the person is allowed to perform masah, wiping their wet hands over the tops of their khuffs. This is a little more convenient and saves a little bit of time.

Scholars are in disagreement as to whether it is permissible to do the same for other socks or shoes, but are unanimous in allowing masah over khuffs. Although the leather socks are the tradition, some Muslims permit masah if they were wearing a form of waterproof shoe. I've seen "modern khuffs" for sale which are just a kind of waterproof sock, and some Muslims say it's ok to perform masah over a regular sock, or if it's cold out, etc..

Khuffs are fairly popular, one can find them in a Muslim marketplace or Islamic clothing store. They're not hard to find online either. Expect to pay around $10US, more or possibly even less in person.

Here is a fatwa discussing the legitimacy of wiping over the socks:

Also, here is a Salafi argument concerning Wiping over the Khuffs: