Can commonly expresses present ability or possibility.
- I can swim.
- We can meet tomorrow.
- Can you help me?
Could can refer to past ability, possibility or a more tentative/polite request.
- I could swim when I was five.
- It could rain later.
- Could you help me, please?
Easy guide
Can = present ability or possibility.
Could = past ability, possibility or a more tentative request.