Ratifying conventions are state-level gatherings where delegates elected by voters meet to approve or reject a proposed constitutional amendment. Article V gives Congress the power to require this method instead of state legislature votes, though it's been used only once: the Twenty-first Amendment repealing Prohibition in 1933.
Congress chose conventions because many legislators believed moral questions affecting individual rights should go directly to the people, not politicians. They also wanted to bypass the powerful temperance lobby that still controlled many state legislatures even as public opinion shifted against Prohibition.
The thirty-eight states that held conventions in 1933 used wildly different procedures—some limited delegate elections to registered voters, others opened them more broadly, and they disagreed on whether delegates could vote however they wanted or had to follow their campaign promises.
Utah's convention cast the decisive thirty-sixth ratification vote on December 5, 1933, making the amendment official just 288 days after Congress proposed it—the fastest ratification in U.S. history.