HR/9 is the actual, non-normalized figure (If I give up 10 HR in 100 IP then just take 10/100x9 = .9 HR/9)
HR/9+ is a normalized number (I"m sure I'm not explaining it quite right), meaning the HR/9 example above is compared to that pitcher's league average for the season in which he pitched. A score of 100 is 'neutral' anything above that means the pitcher gave up fewer HR/9 than league average and anything under 100 means they gave up more. The score can go as high as 9999 for pitchers who give up zero HR.
Using both will ensure we don't have anyone with too low an actual HR/9 (like deadballers) or pitchers who look like they give up a lot of HR, but when normalized actually do not. A modern pitcher with a HR/9 of .75 may very likely be better than league average, and you can tell by his HR/9+ score.
I hope that helps a little without making the true WIS statisticians cringe with my poor explanation.