It means the market values BAC less than what it is actually worth on paper, much less in BACs case. Typically companies repurchase their stock under this scenario, but the banks have had their hands tied for awhile. No more for BAC. The stock price pretty much has to move higher now under the threat of a stock buyback. No one wants to be short BAC right now.
"current stock price divided by trailing annual earnings per share or expected annual earnings per share. ex. XYZ corp. sellsfor 25.50 per share and earned 2.55 per share= ten times earnings" (per NASDAQ glossary-- It is free)