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. The single season mark belongs to George Walker, with a miniscule WHIP of 0.735. The leader among active players is Jacob deGrom with 0.993.