What accusation does Giles Corey make?
I am assuming that you are referring to the place in Act 3 where Corey makes an accusation against someone and not to any accusations that are made against Corey.
In Act 3 of "The Crucible," Giles Corey is trying to get his wife out of the trouble she's in. She's been accused of witchcraft. To try to get her out, he goes to the court and accuses Thomas Putnam of telling his (Putnam's) daughter to "cry witchery upon George Jacobs." In other words, he's saying Putnam put his daughter up to accusing Jacobs of being a witch.
He does this because he believes Putnam is accusing others (including Corey's wife) of being witches so that he will be able to buy their property after they are executed.