The statement means that Google Voice is not a traditional mobile phone service provider like Verizon, AT&T, or T-Mobile. Instead, Google Voice is a voice-over-internet service that allows users to make calls, send texts, and manage their voicemail using an internet connection.
To use Google Voice, you still need a mobile phone service provider for mobile phone functionality. This means you must either continue using your current carrier by setting up a mobile service plan with them or switch to a new carrier. After that, you would generate a new phone number through that carrier. Once you have a mobile phone service plan, you can link it to your Google Voice account to take advantage of its features.