Some application windows are sort of “background noise” and you would usually not like to see them on your taskbar, i.e. on the panel item that shows a button for each application window. Often, applications like that support a “minimize to system tray” option, but some don’t.

In the latter case you might at least want to have a convenient way to hide those windows from your taskbar. I did the following on Debian stable:

Install the “wmctrl” package:

sudo apt install wmctrl

In my XFCE, I added a launcher to the panel, specifying the command:

wmctrl -r :SELECT: -b add,skip_taskbar

See “man wmctrl” for details.