When you bluff pre-draw or pre-flot, the players who put up the blinds are forced to call your bet. If they don't, you get their blinds. I believe that's the "free" pot they're referring to.

But if you bet too much, others will know that you're bluffing and will start calling on hands that they normally wouldn't.

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Note added at 1 hr (2008-12-18 16:49:27 GMT)
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Oh, the reason it's 'free' is because none of your money is in the pot (you didn't put up the blind).

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Note added at 5 hrs (2008-12-18 20:33:49 GMT)
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I thought of another possible meaning of the word 'free' here. I think what it means is that the pot was easy to win, i.e. a freebie, because others had poor hands and did not call your bluff. But if you do it too much, others will know you're bluffing and WILL call with mediocre hands, which may beat yours nonetheless.

Final answer, confidence level 4 :)