is a modal verb used to express possibility or as well as to make suggestions and requests. "Could" is also commonly used in conditional sentences as the conditional form of "can."
Examples:
•Extreme rain could cause the river to flood the city.
•Nancy could ski like a pro by the age of 11.
•You could see a movie or go out to dinner.
•Could I use your computer to email my boss?
•We could go on the trip if I didn't have to work this weekend.
