A symbol in a URL is the SLASH character, which is represented by the letter 'S' followed by the letter 'L' and then the letter 'A' followed by the letter 'S' and finally the letter 'H'. The SLASH character is used to separate different parts of a URL, such as the protocol, domain name, path, and query string. It is also used to indicate the directory structure of a website. The SLASH character is an important part of URLs and is necessary for web browsers to interpret them correctly.