A year ago, Yahoo announced that they would spin off their stake in Alibaba (around 30B). Somewhere along the last year, it was determined that the spinoff might incur a huge tax bill, costing shareholders a few billion dollars. Last month, they decided to do a reverse spin-off. Instead of spinning off Alibaba, spin off everything else (which is only worth a few billion). The tax bill on 4B is much less than the tax bill on 30B.
In other words, all that would be left after the spinoff is an Alibaba holding company.
This creates friction with investors. They'd like access to the value of those holdings without them being used to fund what they see as a losing wager. That's why Yahoo in worth $12b less than the value of their assets.
By spinning off Yahoo, people who don't think it to be a losing bet could invest in it. They think that with time they could turn it around into a sellable asset worth considerably more than if they sold it today. But investors currently don't want to wait for that to happen because they don't like the management team and think they'll just hemorrhage money.