“Red meat” is rhetoric on an issue used to inflame supporters. It is often associated with populist ideas and campaigns.
The phrase was first seen in 1911 in the movie industry, describing movies that were sensationalized. It shifted into a political term in the 1940s.
A quote from the Baltimore Sun shows one of its first uses:
“Most of the audiences… were looking for red meat in Dewey’s carefully reasoned discussions of world affairs. Since he disdained mudslinging they seized upon his withering treatment of bureaucracy and governmental incompetence as a satisfactory substitute.”
Today, red meat is almost always associated with right-wing populist speech. Things that get crowds riled up and angry — such as the “Lock Her Up” chants during the 2016 presidential election — are good examples of red meat.