I found a great definition in a resource guide from the Library of Virginia:
Marriage Bonds: The first law requiring a bond was enacted in 1660/61. It required the perspective groom to give bond at the courthouse in the bride’s county of residence. The bond was pledged, with two or more sufficient securities (or witnesses), but no money was exchanged. A license was then prepared by the clerk and presented to the minister who performed the ceremony. This practice was discontinued in 1849, although in some communities bonds were pledged into the 1850s. Bonding insured against legal action should the marriage not take place, if either party declined to go through with the union, or if one of the parties was found to be ineligible for marriage—for example, if the bride or groom was already married, or was underage and lacked approval to wed.
So the bond did not exchange hands unless something "untoward" came out about either party and the wedding did not actually take place. Keep in mind that the date on a marriage bond is not necessarily the date of the wedding. Though the above definition speaks of Virginian marriage bonds, the basic concept is the same in other states. To read more about marriage records check out this great resource guide by the Library of Virginia.