What is "+" on mouse cursor?

[David Niemi]

I was using the calculator and after I did a quick sum I couldn't close
any open windows and noticed a small "+" sign attached to my mouse
arrow / cursor.  My keyboard is somewhat responsive but I cannot close
or select other open windows with the mouse and the keyboard shortcuts
somewhat work.

I've never seen this before nor read about this, any ideas on what I can
do to get control back or should I just force reboot?

TIA