The activation requirement of a card is a condition that must be met in order to activate that card. On cards with Problem-Solving Card Text, it is written before a colon, which is then followed by the cost (if any) and effect.
- The activation requirement of "Magic Jammer" is the activation of a Spell Card.
- The activation requirement of "Mirror Force" is your opponent declaring an attack.
- The activation requirement of "Fuh-Rin-Ka-Zan" is controlling at least 1 EARTH, 1 WATER, 1 WIND and 1 FIRE monster
Usually, the conditions need only be met at the time of activation for a card to resolve; even if at the time of resolution the conditions are no longer valid, the card still resolves, unless it has the text "(condition must be true) to activate and resolve this effect".
Player A controls "Elemental Mistress Doriado" and activates "Fuh-Rin-Ka-Zan". Player B chains "Raigeki Break" to destroy "Elemental Mistress Doriado". Even though by the time "Fuh-Rin-Ka-Zan" resolves its activation requirement cannot be met, it still resolves because the requirement was met when the card was activated.