Your public IP address is the external facing IP Address that is provided by your Internet Service Provider (ISP). A Pubic IP Address is accessible by anyone on the Internet. However, it may not always be accessible by everyone. The access may be restricted by your ISP in certain cases based on the package you have obtained.
Knowing what your external facing IP address is critical for opening ports used for server services, gaming, running an email or web server, or setting up a remote connection.