Ophelia tells her father exactly what Hamlet expected she would tell him. Hamlet is using Ophelia; he wants King Claudius to start being nervous about him, and he knows that if he acts weird to his girlfriend, Ophelia, she will tell her father, Polonius. And Hamlet knows that, since Polonius is an advisor to the King, he will straightaway tell Claudius everything Ophelia has told him.
Of course you could say, Why doesn't Hamlet just go kill Claudius? Why does he take this roundabout approach to revenge? Well, that a question for another time. Let's just say, for now, that Hamlet is a schemer and a planner and a thinker, more than he is a doer. Indeed, he would rather act at doing something than really do something.
Here, then is all of what Ophelia relates to her talkative old man about Hamlet's behavior, his wordless conversation with her, (from Act 2, Scene 1):
My lord, as I was sewing in my closet,
Lord Hamlet, with his doublet all unbraced,
No hat upon his head, his stockings fouled,
Ungartered, and down-gyved to his ankle;
Pale as his shirt, his knees knocking each other,
And with a look so piteous in purport
As if he had been loosed out of hell
To speak of horrors, he comes before me...
...He took me by the wrist and held me hard;
Then goes he to the length of all his arm,
And, with his other hand thus o'er his brow,
He falls to such perusal of my face
As he would draw it. Long stay'd he so.
At last, a little shaking of mine arm,
And thrice his head thus waving up and down,
He raised a sigh so piteous and profound
As it did seem to shatter all his bulk
And end his being. That done, he lets me go,
And with his head over his shoulder turn'd
He seem'd to find his way without his eyes;
For out o' doors he went without their help,
And to the last bended their light on me.
Oh, how very dramatic stuff! But it's just a little play acting on Hamlet's part. We will see later in this act and especially in Act 3, how fond Hamlet is of plays and players.
Yes, in a lot of ways Hamlet is a play about playing at acting without actually acting.