Push. That is what push is. It means that changes are sent as they happen.
"Email accounts that use the push method for message retrieval enable you to have email messages delivered directly to you as soon as the message is received by your email provider instead of checking periodically or manually for new messages. On iPhone or iPod touch, push email is supported for the following email account types:
MobileMe
Microsoft Exchange
Yahoo!
Other email accounts such as POP, IMAP, AOL or Gmail accounts do not support push message retrieval on iPhone or iPod touch but can instead be setup to fetch mail at specified intervals or manually when the Mail application is opened."
http://support.apple.com/kb/HT2625