In less severe cases when you're just a little dehydrated you might experience muscle cramps, or feel faint and tired.
In severe cases you can die of dehydration. Most times though you just need to increase your fluid intake to overcome dehydration.
You should always drink something before you actually get thirsty to stay hydrated, and that's especially true during really hot weather since during the hot weather we sweat out more fluid and that fluid needs replaced more often to avoid dehydration and getting heat stroke.