When plants get "wilted", it means that they become too soft. For example, if you can squeeze a carrot and it feels soft on the outside, it's "wilted". Lettuce is wilted if the leaves fall over instead of standing up when you hold one up. Being "wilted" is almost always a bad thing for vegetables.
Some vegetables aren't often described as "wilted", like tomatoes, potatoes, or onions. Usually long, thin vegetables or leafy vegetables are described as "wilted".
Plants can wilt as well as vegetables. Plants usually wilt when they haven't been given enough water.