'Could' can be used to talk about the past, the present or the future. 'Could' is a past form of 'can'.
When I was living in Boston, I could walk to work.
He phoned to say he couldn't come.
I could see him clearly but I couldn't hear him and then the videoconference line went dead.
'Could' is used to make polite requests. We can also use 'can' for these but 'could' is more polite.
Could you help me, please?
Could you lend me some money?
Could I have a lift?
Could I bother you for a moment?
If we use 'could' in reply to these requests, it suggests that we do not really want to do it. If you agree to the request, it is better to say 'can': Of course, I can.
I could help you if it's really necessary but I'm really busy right now.
I could lend you some money but I'd need it back tomorrow without fail.
I could give you a lift as far as Birmingham.
'Could' is used to talk about theoretical possibility and is similar in meaning to 'might'.
It could rain later. Take an umbrella.
He could be there by now.
Could he be any happier?
It could be Sarah's.