It is the ability that raises your attack power by +1. But Raist is correct: always do something! If you start a battle in an already tactically sound position, I see no reason why you couldn't spam 'Focus' and wait for the monsters to arrive.
On the subject of secondary abilities, useful combinations can very advantageous. Fundaments, the Squire abilities, give you Focus, which is immediately useful.
Another would be to equip an Archer with the Arts of War secondary ability from the Knight. Arts of War deals with stat decreases and the breaking of armour to cripple your foe. What better way to do that than with a long-range ability! By the time they approach, they will have been weakened enough for your front-liners to pound them. Experiment with different ability combinations, and see what works. Items is always a safe option, as mentioned previously. A Black Mage with another magical secondary ability is a very powerful force, especially summon: all summon spells do not have friendly fire and target only who they are meant to target.