Using Push allows emails to be received even when you’re not using the Mail app. Keeping push email turned off can save quite a lot of juice though, particularly in areas with poor signal.
To turn it off, go to Settings, select the Mail option, then Fetch New Data. If you have multiple email accounts set up this will be found in the Accounts section. Turn off Push at the top of the screen then scroll down and set Fetch to Manually.
To receive email with Push switched off, just open the Mail app.