Can someone please explain to me what the statistic "WHIP" means in the game of baseball?
The definition of WHIP or Walks plus Hits per Inning Pitched is the following per Wikipedia:
WHIP = (Walks + Hits)/(Innings Pitched)
In addition to the popular answer above, it's also useful to note that WHIP was originally conceived of as a statistic used to score pitchers in fantasy baseball.
So while WHIP is not widely used in sabremetrics communities to help rank the usefulness of pitchers, it is a useful statistic to understand in fantasy circles as well as use as an indicator of future ERA performance. Ex: Lower WHIP generally coincides with lower ERA. Since ERA is such an established stat (for legitimate reasons or not), WHIP can be used as a tool to forecast future performance of a pitcher's ERA.
WHIP stands for "walks and hits (allowed) per inning pitched." It is a measure of the rate at which a pitcher allows opposing batters to get on base.
I don't quite agree with another answerer who says it's a "poor tool" to evaluate pitchers. The "walks" or W part of the equation are entirely under control of the pitchers. What the other person meant was that HITS aren't regarded as entirely under the control of pitchers, because of fielding issues.
There are two more items that define a pitcher. Strikeouts aren't explicitly included in the WHIP formula, but they feed indirectly into the "hits" part of the equation; the more strikeouts, the fewer opportunities for hits.
The last thing is home runs, (or other extra base hits). All other things equal, a pitcher with a lower WHIP than another will give up fewer runs, but a pitcher with a low WHIP and a high home run rate (e.g. the Pittsburgh Pirates' Edinson Volquez) might still give up a lot of runs.