The TED spread is a gap between two interest rates, which is used as a marker of the financial strength of banks.
The treasury bill rate is the interest rate paid by the U.S. treasury - often used to represent "risk-free" lending (on the assumption that U.S. government is always good for it), while the LIBOR is the rate at which banks lend to each other. Therefore, the difference in the two rates represents the "risk premium" of lending to a bank instead of to the U.S. government. At its lowest, the TED spread can be as low as 20 basis points, as it was in early 2007. A TED spread this low occurs when banks are seen as strong and in good financial health; the risk of default or banktruptcy is low, and therefore other banks are willing to lend them money at nearly the risk-free interest rates paid by the U.S. government. By contrast, the Ted spread stood at 330 basis points in early October 2008, after a series of bankruptcies by banks and other financial insitutions that occured as part of the 2008 Financial Crisis. On October 10th, the TED spread hit a new record of 460 basis points, reflecting a breakdown in interbank lending.
Looking at TED Spread
on Reuters 3000 Xtra: TED
on Bloomberg Terminal : .TEDSP <INDEX> <GO>
on Bloomberg Website :  .TEDSP:IND