Fetch New Data is a feature that allows the user to select how often the device checks for new email. Push is a feature that's only available with certain email accounts that automatically push email to the device.
Push requires more power and may have an effect on battery life.
Depending on your software version, from a Home screen, do one of the following:
Navigate: Settings > Mail > Accounts.
Navigate: Settings > Mail, Contacts, Calendars
Navigate: Settings > Accounts & Passwords
Navigate: Settings > Passwords & Accounts
Tap Fetch New Data.
Tap the Push switch to turn on or off .
Select an email account.
Select the schedule option then tap Fetch New Data (upper-left).
The schedule option is selected when a check mark is present.
Select a Fetch setting:
Selected when a check mark is present.