It refers to when the memory gauge is at 4 or further to the right on your side.

It refers to a card that contains the specified text or icon in its name, traits, effects, inherited effects, (Rule), digivolution requirements, DNA digivolution, DigiXros requirements, burst digivolve, App Fusion, Link, or Assembly requirements. For example, a card with [Knightmon] in its text would include cards with the name [DarkKnightmon] and cards with the text [Knightmon] in their effects.

1 [Asuna Shiroki], 1 level 4 or lower Digimon card with the [TS] trait, or 1 level 4 or lower Digimon card with [Three Musketeers] in its text.

No, you can't activate it.