Advanced Gameplay FAQ: Activation Requirement

The Activation Requirement of a card is a payment or condition that must be paid and/or met, in order to activate that card.

Examples
The Activation Requirement of "Premature Burial" is the payment of 800 Life Points.
The Activation Requirement of "Magic Jammer" is to discard one card from your hand.
The Activation Requirement of "Mirror Force" is your opponent's declaration of an attack.

The Activation Requirements of "Radiant Mirror Force" are your opponent's declaration of an attack and controlling three or more Attack Position monsters.
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need only be met at the time of activation for a card to resolve; even if at the time of resolution the conditions are no longer valid, the card still resolves (unless negated by another card effect).

Example breakdown
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.