It doesn't apply for you at this point.

The existing number thing is as follows: if you don't have a Google Voice account, you can tell it you want to join using your existing cell phone number. The moment you've done so, nothing has actually really changed. Any calls to or from your phone are still going over your carrier's network and has nothing to do with Google.

In order to utilized Google Voice with your existing number after you've signed up as such, there's two things you can do:

1) follow the Google-provided instructions for setting up GV as your voicemail service. This involves dialing a special number that changes the forwarding number for missed calls from your carrier's voicemail system to GV's.

2) place calls from GV. Using the web interface, Chrome extension, or the GV app on Android its self, you can place a call originating from GV. This, as I've mentioned before, still uses your minutes, but it's VOIP from Google to the recipient. If using your existing number, the ONLY reason this is useful is to place inexpensive international calls.

That's pretty much it! If you sign up for GV with your existing number, you just don't get the ability to ring multiple phones. It's like a GV-Lite account.

*edit*
Actually, I need to correct myself. "Use with existing phone number" has one benefit for existing GV customers who already have a GV phone number. It brought the ability to forward calls to my cell phone number (as opposed to my GV number) to my GV voicemail system. So whether someone calls my cell phone or my GV number, it all goes to the same voicemail inbox, and I still get all the benefits of GV like personalized greetings and visual voicemail.