Source: Yugioh Wikia
Link Materials are monsters that are used for the Special Summon of a Link Monster. They must be face-up on the field, even if the Link Monster does not specify. They are listed in the first line of a Link Monster's text.
Normally a monster can only be treated as 1 Link Material, but in the case of Link Monsters, they can alternatively be treated multiple Link Materials, equal to their own Link Rating.