The cobbler asks Marullus not to be out with him, which means he doesn't want Marullus to be angry with him. But when he continues, "if you be out, sir, I can mend you", Marullus thinks that the cobbler is being insolent in saying that he's going to set Marullus right by getting rid of his anger. However, in the cobbler's trade, the expression "out at heel" refers to a shoe so worn that the heel is protruding from it. The double meaning here means that the phrase could just be an offer by the cobbler to mend Marullus' shoes if they need it.