D&Dtools1

Benefit

As a standard (readied) action, you can expend one or more turn or rebuke undead attempts to counter another caster's spell, as though you had cast the same spell yourself (see Counterspells, PH 170). You must spend a number of daily uses equal to 2+ the level of the spell you wish to counter (minimum l) and be able to cast that spell as if it were on your spell list.

Normal

You can counter a spell only by casting the same spell, a specific counterspell, or dispel magic