Wikipedia says this about the Manchin-Toomey Amendment:
The Manchin-Toomey Amendment was a bi-partisan piece of legislation that would require background checks on most private party firearm sales, sponsored by Democratic Sen. Joe Manchin and Republican Sen. Pat Toomey. The amendment, S.Amdt. 715 to S. 649, was voted on and defeated on April 17, 2013 by a vote of 54–46. It needed 60 votes to pass.
What law was this actually amending? Why did it need 60 votes to pass?