MACE stands for "major adverse cardiovascular events." It is a composite endpoint — a bundle of outcomes counted together — that cardiovascular trials use as their primary measure. The standard "3-point MACE" combines cardiovascular death, non-fatal heart attack, and non-fatal stroke; some trials add hospitalization for unstable angina or heart failure to make a broader version.
When you read that a GLP-1 "reduced MACE by 20%," it means participants on the drug were 20% less likely to have any one of those events over the trial than participants on placebo. Semaglutide (SELECT), liraglutide (LEADER), and dulaglutide (REWIND) have each shown MACE reductions, and tirzepatide was shown non-inferior to dulaglutide on MACE in SURPASS-CVOT. See the cardiovascular benefits of GLP-1 medications.