This is the accusative case, which shows who or what is it that's directly affected by the action.
In English you mainly use the word order to express this: The dog bites Peter isn't the same as Peter bites the dog.
Hungarian on the other hand uses a -t suffix to express this, so the word order doesn't matter. So using the same example: A kutya megharapja Pétert and Pétert megharapja a kutya both express that it's Peter who gets bitten.
And it just happens that keres is a verb that requires the accusative case, unlike its English counterpart which requires the preposition for. Differences like this simply have to be memorised.