iPad Mini Mail App. What are the icons on the top of the Mail app for?

[iMore Question]

There are 5 icons across the top of the reading pane. It is the middle one is the one in particular that I need advice on. I have an iPad Mini I just started using. I have a Google email account set up using the Mail app. While reading email I touched an icon on the top bar and the email I was reading disappeared. I didn't finish reading it. I has not disappeared from my email just the iPad. Also is there any controls to how I view the email (i.e. size of reading pane)?

 

[Old Soldier]

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