Employee Stock Ownership Plan.
Real cash is traded for worthless pieces of paper. Usually u can't spend the paper for 20 years or so. There were lots of "Millionairs" at people's express for about 2 weeks before the stock tanked. The united pilots cant sell their shares till they retire, but in the meantime others are retiring and selling their shares, further diluting the power of the pilots
It is a VERY bad deal, that looks like a great deal.
Cheers
Wino