A white elephant is something that is presented as a gift or a benefit, but turns out to be a bad thing -- usually because it's expensive. For example, a government might introduce some reforms to help small businesses, but to finance the reforms business taxes have to be raised, forcing lots of small businesses to close down. That would be a white elephant.
The phrase probably has its origins in a story involving a ruler somewhere in India. He would sometimes give as a gift a white elephant to a subject he didn't like. The subject would be forced to accept the gift, as it would be a terrible insult to refuse; but elephants are very expensive to keep, and so the unlucky recipient would soon find himself penniless.
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