Some beings have psychic powers — that is, they can tap into the alternate dimension of the Warp and direct a little of its fickle energy on the battlefield and elsewhere. These beings are known as 'pyskers'.
Some psychic powers are very subtle, but those that manifest on the battlefield tend to be spectacularly violent: a sudden conflagration of all-consuming spectral fire, a powerful bolt of dark lightning, a blast of telepathy that can bypass armour and melt minds.
Where your unit has the Psyker ability:
You can use certain Fate cards to make a special Psychic Blast attack with the unit.
You can use certain Fate cards to try to nullify a Psychic Blast attack by an enemy Psyker unit.