The word "could" is a modal verb used to express possibility or ability in the past. For example, if someone says, "I could swim when I was five," it indicates that they had the ability to swim at that age. It can also suggest a hypothetical situation, such as, "We could go to the park if it stops raining." In addition to expressing ability, "could" is often used to make polite requests or suggestions. For instance, saying, "Could you please help me?" is a courteous way to ask for assistance. This usage makes "could" a versatile word in everyday conversation.