A Legend Material (Legenddo Sozai) are cards that are needed to Legend Summon a Legend Monster. When a Legend Monster is summoned, Legend Material cards are placed into the Legend Gauge.
Legend Material can be a range of anything from very generic (Effect/Normal Monsters) to requiring specific properties (Level/Rank, Type, Attribute, or even Archtype). Legend Materials are also usually not a specific number. It's usually a range of numbers (ex. 2-3, 2 to 3 monsters). Stronger Legend Monster can require more specific materials with smaller ranges, while weaker monster can require less specific materials with larger ranges.