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Yeah, it's Walks + Hits divided by Innings Pitched. Basically, in a nutshell, it's the ratio of baserunners allowed to innings, so a 1.20 whip allows an average of 1.2 baserunners per inning. So it penalizes a guy who maybe allows the bases to be loaded, even if he gets out of it without giving up any runs.
And yeah, around or under 1.00 is very good for a starter. But like ERA, the top relievers are usually a bit better, since they throw fewer innings.
League average WHIP in Major League Baseball last year was somewhere around 1.35, I believe.