In King Lear, what does Cordelia mean when she says to her sisters in Act 1 scene 1, "I know what you are"?
Whenever you are trying to focus on what a specific quote from a text means it is important that you do two things. Firstly, you must consider what the character says before and after the quote, and secondly you must consider the meaning of the quote in the light of the context of that scene or chapter as a whole.
Cordelia therefore says this quote to her sisters after she has been exiled by her father for not testifying with sufficient eloquence to the love that she has for Lear. Having heard Goneril and Regan exaggerate and lie in the hope of gaining more land, Cordelia shows her love with simplicity and not with insincere hyperbole. As a result, she receives nothing except being cast out of her father's kingdom. Cordelia, however, clearly sees through her sisters' plot as this speech suggests:
The jewels of our father, with washed eyes
Cordelia leaves you. I know you what you are,
And like a sister am most loath to call
Your faults as they are named.
The quote you identified signifies the way that Cordelia understands the faults of her sisters and the game that they are playing. However, being too good a character, she will not denounce them, only leaving saying to them:
Time shall unfold what plighted cunning hides.
Who covers faults, at last shame them derides.
Thus the quote you have identified signifies the animosity that there lies between Cordelia and her sisters and likewise testifies to the honesty of Cordelia's character.