Shylock's deal with Antonio is to take a full pound of Antonio's flesh (no more no less)if he defaults on his loan. Shylock, of course, is hoping that Antonio defaults so that he can extract his payment and settle the bond.
If Shylock takes too much flesh or too little flesh, then his bond will not be fulfilled and might be held responsible for Antonio's death or suffering. On apractical level, if Shylock takes too much flesh Antonio might lose too much blood or even die. If he takes too little flesh, then Shylock's cutting might cause infection which could also lead to death. It is difficult to fulfill the bond either way.
Shylock's deal is destroyed when the disguised Portia argues that Shylock may have a pound of flesh, but not a drop of blood.