There's some debate as to whether or not this actually helps preserve your battery life, but Cult of Mac makes the case for how switching from "push" to "fetch" could cut down on power usage. With the "push" feature turned on, your phone is constantly listening for new emails and downloading them from the server in the background.
The "fetch" setting, however, tells your phone to ping the server every once in a while to download new emails, meaning your phone isn't always wasting energy by downloading emails in the background. To change this, go to Settings > Mail, Contacts, Calendars > Fetch New Data, and make sure the Push switch is off.