According to
http://www.saremba.de/chessgml/standards/pgn/pgn-complete.htm#c8.2.4
it is a "Numeric Annotation Glyph" and the different number codes stand for things like Black/White advantage, good/bad piece placement, time control pressure etc. They are listed in Table 1 of the above document.
When looking at engine results, I'm curious as to the meaning of the $## at the end....
(23. Qf3 Qxd7 24. Rad1 Bd6 25. Rfe1 Rb8 $19)