From Yu-Gi-Oh!

Quick Effects (previously known as Multi-Trigger effects) can be activated either during your turn, or your opponent's turn. They are the only Monster Effects that are Spell Speed 2.

Monsters that have quick effects usually have: "This effect can be activated during either player's turn", or something similar in its card text. Also, there are some exceptions of monsters with quick effects which don't read such effect in their lore (like "Horus the Black Flame Dragon LV8").

Some Quick Effects can be activated more than once in a chain, while others cannot.

• Example #1: "Horus the Black Flame Dragon LV8" exists on the field, your opponent activates "Fissure". Horus's effect immediately activates since "Fissure" is a spell card. Your opponent replies by chaining "Book of Moon", and again, you can activate the effect of "Horus the Black Flame Dragon LV8" because it has a Multi-Trigger Quick Effect.

• Example #2: "Light and Darkness Dragon" exists on the field and your opponent activates "Fissure". "Light and Darkness Dragon's" effect activates as a chain to Fissure. Your opponent replies by chaining "Book of Moon", but because "Light and Darkness Dragon" has a Trigger Quick effect rather than a Multi-Trigger Quick effect, it cannot be used more than once in a chain and therefore cannot negate "Book of Moon's" activation.

see List of Quick Effect Monster Cards

Example