Help with Snapchat

[Namechangeforthisonenow]

What's these boxes and arrows mean?

It means you have opened two of their snaps (the top two) the arrow one when they’ve opened your Snapchat.

Why does it stay there for a while?
I'm very new to Snapchat! As you can probably tell

I think it’ll only stay there until you click on the chat and then it’ll go. It just shows you what snaps have been sent/received lately. Once you’ve opened/sent the snap it’s gone forever, unless you or they screenshot it. (But it’ll tell you they’ve screenshot it)