When you extend a Web Application A it will create a new IIS Site with a different port number but will use the same Content Database of Web Application A. This way both sites will be serving the same content but will be having a different URL.
The difference in URL is something the end users will experience. But the difference and the use of web application extension is more than that. Say for instance we have a Web Application that uses Windows Authentication based on the company's Active Directory. But now you want the same web application to be exposed to external users outside the company.
In such a case you can extend the web application and use a different authentication mechanism like Forms Based Authentication where the external users are maintained in a separate database other than Company AD and uses an authentication mechanism other than the normal Windows Authentication.
Ultimately it will grant access to same Content Database.
To sum up your questions :
- What does it mean by serves the exact same content as other websites in your web application ? It means the Old WebApplication URL and the extended web application URL will refer to the same Content Database
- Does it mean that when I extend a Web application to a new zone, for example Internet, it creates a new Site Collection ? It does not create a new site collection. The main implementation lies in the fact that the new IIS Site will have its own web config file and it knows which authentication mechanism to use and what content to serve.