what's this symbol mean ( square with 3 curved lines) The leftmost icon(rectangular) on above screen shot showed up yesterday. I don't know, where or what app it's for but I would like it gone.
This symbol is of "Cast screen" found in Android which is used to cast your screen to a television. This symbol in the status bar means that your phone screen is being casted to a television or cast screen is active:



If you want to get rid of it, just go to settings and disable screen casting.

More info:
Google Support