The ampersand in the end of this command tells this command will be executed in the background. After starting a command, you can press Ctrl+Z to suspend it, and then
bg to put it in the background if you want.
This way still command output and command errors will show up in the terminal. But you can try this:
long-running-command > /dev/null 2>&1 &
With the last line you will not get the standard output (/dev/null ) and the errors will be redirected also to the standard output meaning will not show.