The Simple Answer:
Usually not needed, but on some occasions, collateral is security you put up that can be liquidated, or cashed in, if the defendant does not show up and the bail company has to pay the court the amount of the bail bond.
The Full Answer:
If the defendant were not to show up in court, the bail company has 6 months to get the defendant "right with the system" or they have to pay the value of the bond. That could get expensive pretty quick, so bail companies will sometimes take collateral to secure a defendant's presence in court. Suppose someone doesn't show in court and the bail company can't find them, in 6 months the bail company will liquidate any collateral in order to pay the courts the face value of the bond, on the other hand, once a defendant's business with the court is over and the bail bond is exonerated, all collateral is 100% refundable. Cash, Real Estate, Stocks & Bonds, Life Insurance Policies and vehicles are all acceptable forms of collateral.