Both can and could are modal verbs, that are used to express the possibility of the ability of doing something, but the difference between them, that Can is used to express a general truth or something with a high chance of possibility, while could is use when you speak about something that is weak to happen, or something, that is not necessarily a general truth, also both can and could are used to make a request or ask for a permission, but Could is more polite.
Examples: The horse can run quickly.
He could buy this car, if he had enough money.
