The Gemara says that if he made a Neder against his son-in law, and he wants to give something to his daughter. However, technically, anything given to the daughter belongs to her husband. So he needs to say that he's giving money on condition that her husband doesn't receive it and he gives it to her to eat.
Ran explains: since it's only given to her to eat, she only acquires it at the time that she's eating. At the point that she's swallowing it, it cannot revert to the husband, so it never got to the husband's pocket. However, this would not help by clothing, since she acquires it when she wears it, and then it would revert to the husband's property.
However, some explain: since he only gives it for one purpose and she cannot exchange it for another purpose, she never makes a complete acquisition on it. Since she never completly owns it, it never revert to the husband's property.