It means that your iPhone must be set to "fetch" GMail. That means that every 15 minutes, 30 minutes, or every hour (depending on how you set it up), the iPhone will poll your GMail account and check for new mail. Or, you can set it to retrieve "manually" (that is, it checks for new messages when you open up the mail app).
This is opposed to "push," which pretty much means a new message will show up on your iPhone's inbox moments after it's received by GMail.
GMail has a push capability, but has not enabled it on the iPhone for whatever reason. Right now on the iPhone, push e-mail is supported by Yahoo, Mobile Me, and MS Exchange mail servers.
For what it's worth, it looks like the situation was pretty similar with GMail on Blackberries using BIS. RIM describes synchronization as "
near real-time", and that it uses IMAP just like the iPhone does. So I would say you're probably not going to notice too much of a difference.
RIM also notes their sync with Gmail is "one-way...from the BlackBerry smartphone to the Gmail webmail account, of read and unread status, sent items, and deleted items." Which may mean that read/unread status isn't update on the Blackberry if you happen to read a message using the webmail interface. So I'm guessing it's not as seamless on RIM as you would think.