Q: What is beside my dock?

I have no idea how to ask this question because I don't know what it is, but I keep getting this little popup of the Mail app and the Calendar app on the left hand side of the dock on my iMac.  What is it and how can I make it stop?  It's very annoying and the only thing it does when I click on it is open the application. (See left side of screenshot there's the Calendar app that keeps appearing then switching to the Mail app then disappearing altogether)