This occurs in the midst of a conversation between Sir Toby Belch, Maria and Sir Andrew. They have been bantering and Andew is busily revealing himself to be a fool. Andrew has just asked her if she thinks she has fools in hand, to which she replies, "Sir, I do not have you by the hand." A wiser man might have been insulted. Toby Belch swoops in with a further play on the word, saying "Marry, but you shall have and here's my hand." By saying this, he accepts the label "fool" and defuses the insult she's laid upon Sir Andrew, and also adds another level: flirtation. To take someone by the hand is to lead them; Sir Toby Belch is saying he wishes to be led by Maria. Underneath all of this is the further meaning--to offer one's hand is to offer oneself in marriage. Sir Toby is definitely not doing this here, but is probably quite aware of the connotation of the words.