It is a polite way to ask someone to do or give something, a softer version of can. Could you pass the salt? Could you email me the file?
In other sentences, it can mean past ability: I could swim before I turned five.
You're at your desk and need a colleague's help. You might write, Could you send me that report by Friday? The sentence asks for something, but gently, leaving the other person room to say no or suggest a different day.
Or someone short of change at a shop counter: Could you lend me £5? I'll pay you back tomorrow. Same polite request shape, this time asking for something rather than asking someone to act.
Could and can both ask someone to do something here. The difference is tone: Could you open the window? sounds a little softer and more polite than Can you open the window?, which is fine but more casual. Could leaves slightly more room for the other person to say no.
A nearby mix-up worth ruling out: this request meaning has nothing to do with the past-ability use of could. In Could you help me? no past time is meant; it is a polite question about right now.