From BR Bullpen

WHIP stands for walks plus hits, all divided by innings pitched. It is one of the standard measures of a pitcher's efficacy, noting how well they prevent baserunners. It became prominent in the 1980s and 1990s, its used pushed by fantasy baseball, especially "Rotisserie"-type leagues, which used it as a category to rank pitchers.

The best WHIP of all time (minimum 100 decisions) is owned by Hall of Famer Addie Joss (0.968). Any number below 1.00 is considered exceptionally good.