To activate is to use a Card or Effect also known as playing a Card.

Activate and Use are not the same if used in Card texts.

Spell Speed
All activated Cards and Effects have a Spell Speed. You cannot activate Cards and Effects in response to Cards or Effects with a higher Spell Speed. In addition, you can’t activate Cards and Effects while a Chain is resolving.

Play a Card
To activate a Spell or Trap Card you have to play it from your hand or flip it face-up if it was set.

Apply an Effect
To activate an Effect is to choose to apply a Trigger or Trigger-like, Ignition or Ignition-like, or Quick or Quick-like Effect. When activating an Effect from a location that is not public knowledge, the Card that activated the Effect is revealed by game mechanics.

Distinction between Card and Effect
Activating a Card is distinct from activating its Effect. For example, "Magic Jammer" and the Effect of "Dark Paladin" can only be activated when the Card "Supply Squad" is activated, but it cannot be activated when the Effect of "Supply Squad" is activated.

Activation requirements
There are several requirements that must be met in order to activate a Card.

You can only activate a Card or Effect if that Card or Effect has met its activation conditions. However, some Cards do not have any activation conditions.
For example, you cannot activate "Dark Magic Attack" if you do not control a "Dark Magician". However, you can activate "Harpie's Feather Duster" because it has no activation conditions.

You have to pay the activation cost for a Card or Effect in order to activate a Card or Effect, but many Cards do not have any cost.
For example, you can’t activate "Twister" if you cannot pay 500 LP (for instance, if you have less than 500 LP). But "Mystical Space Typhoon" can be activated because it has no cost.

It is not possible to activate a Card or Effect if the Effect would fail to do anything when it resolves.
For example, you cannot activate "Lightning Vortex" if your opponent does not control any face-up Monsters.

It is not possible to activate a Card or Effect if it has no valid target.
For example, you cannot activate the Effect of "Zombie Master" if there are no Zombie-Type Monsters in either Graveyard that can be Special Summoned.