Most people who tie a string on your car door handle are trying to send you a message. The string symbolizes the connection between you and the person who tied it there. This person wants to talk to you about something, but they don't want to do it in person. They're hoping that by leaving this string, you'll know that they're thinking about you and that they want to talk.
There are a few different ways that people interpret the meaning of a string on a car door handle. Some people believe that it's good luck, while others see it as a way to ward off evil spirits. Whatever the reason, tying a string on someone's car door handle is usually an act of goodwill.