By learning even the most basic principles of an Arcanum, a mage understands how to counter a spell.
Counterspell is a Clash of Wills, pitting the acting mage’s Gnosis + Arcanum against the countering mage’s Gnosis + Arcanum.
A mage can attempt to counter any spell that uses the Arcanum, even if it uses other Arcana as well, but always counters the highest Arcanum of a target spell. The comparative ratings of the two mages’ Arcana are irrelevant; an Initiate can, in theory, counter the spell of a Master.
Countering the spell of a mage with a higher rating in the target Arcanum, however, requires that the player spend a point of Mana.
Counterspell requires an Instant Action. If the mage is employing Active Mage Sight, she can attempt to counter a spell of the appropriate Arcanum in combat, regardless of her position in the Initiative order, provided she has not used her action yet.
Knowledge of one Arcanum imparts the understanding of how to unravel it. To cast a spell, the mage forms an Imago; to counter a spell, the mage simply needs to disrupt one.
The Counterspell Attainment is actually 10 different Attainments, one for each Arcanum. By learning even the most basic principles of an Arcanum, a mage understands how to counter a spell.
By the time a spell takes effect and a mage feels it in Peripheral Mage Sight, though, it’s too late to counter; to use this Attainment, the countering mage must see her rival casting in Active Mage Sight.
Example
A Moros mage is using Active Death Sight during a fight with some Seers of the Throne.
She sees that one of them is about to cast a Death spell at one of her compatriots. Even if the Seer acts before her in the Initiative order, she can attempt to Counterspell.
If the Seer had been readying a Forces spell, the Moros would have noticed it, but could not attempt to counter it with Death (she could, however, use Universal Counterspell if she had Prime 2).