The Shabbat prohibition of Borer forbids separating between desirable and undesirable items, such as between foods which one wants to eat and foods which he does not want to eat. Separating is allowed on Shabbat only if one removes the food he wants to eat from the food he does not want to eat (and not the other way around), and he does not use a utensil for this purpose. The third condition is that the separating is done for immediate purposes meaning, to eat the food right away, rather than for later in the day.
It occasionally happens that a person is served a bowl of kibbehamda soup with meatballs and vegetables but he only wants the vegetables and meat, without the broth. A person in this situation might decide to take a ladle, fill it with kibbehamda, and then tilt the ladle so that the broth spills back into the bowl, leaving only the solid food. This would be forbidden on Shabbat, because this entails removing the unwanted food the broth from the food which one wants the vegetables and meat. He would have to instead simply take what he wants from his bowl, and leave behind what he does not want.
Another common situation which many people do not realize can entail a violation of Borer is washing grapes. Many people place grapes in a bowl, fill the bowl with water, and then spill the water into the sink while holding the grapes inside the bowl. This, too, violates the prohibition of Borer, because one separates the water which he does not want from the grapes the food which he does want. One would have to remove the grapes from the water and put them somewhere else, rather than separating the water from the grapes.
Summary: If one wants only the solid food in his soup, such as the vegetables, he must remove the vegetables from the broth; he may not tilt the spoon or ladle to remove the broth from the vegetables. If one fills a bowl of grapes with water to wash them on Shabbat, he must then remove the grapes from the water; he may not pour the water out of the bowl, separating the water from the grapes. It is forbidden to use on Shabbat a perforated spoon made to remove liquid from food.