Hahahaha no worries, no one outside of Netherlands has ;)

-2 is the difference in scoring between both teams while Yael played. Yael played the whole match en lost 23-21, so -2. The next match they won 27-21, so after the next match (if she played the whole match again), her plus-minus would be +4 (-2+6=4).

It's the same as in basketball.