A Fusion Summoning condition is a restriction placed on a Fusion Summon that does not apply to most such monsters.
All Fusion Monsters that do not include contrary Summon conditions must be Fusion Summoned properly first or else they cannot be Special Summoned again from the Graveyard or Banished Zone. Two such conditions exist.
The first uses the wording "This card cannot be Special Summoned except by Fusion Summon." This is primarily found on "Elemental HERO" monsters. It means that even if you Fusion Summon the card correctly first, you cannot Special Summon it again afterward. This also means you cannot use cards such as "Metamorphosis" or "Cyber-Stein" to directly Special Summon the Fusion Monster(s) from the Extra Deck.
The second states that "A Fusion Summon of this card can only be conducted with the above Fusion Material Monsters." This means that for a Fusion Summon to take place with the card you must use the Fusion Material Monsters listed on the card-you cannot use Fusion Substitute Monsters or cards like "Instant Fusion". However, if there are no further summon conditions, you can still Special Summon the monster directly from the Fusion Deck with cards such as "Metamorphosis" or "Cyber-Stein". You may also use cards whose effects change their names such as "Elemental HERO Prisma".