Glaciers retreat when their terminus does not extend as far downvalley as it previously did. Glaciers may retreat when their ice melts or ablates more quickly than snowfall can accumulate and form new glacial ice. Higher temperatures and less snowfall have been causing many glaciers around the world to retreat recently
Muir Glacier, located in Glacier Bay, Alaska, photographed by W. Field in Aug. 1941 (left) and B. Molnia in Sep. 1976 (middle) and Aug. 2004 (right). Note how the glacier has retreated to expose rock in 1976 that has since become lush vegetation in 2004. The glacier has retreated so much that it is hardly visible in the 2004 photo. —Credit: National Snow and Ice Data Center (comp.). 2002, updated 2009. Glacier Photograph Collection. Boulder, Colorado USA: National Snow and Ice Data Center
https://nsidc.org/cryosphere/glaciers/gallery/retreating.html
https://nsidc.org/glims/glaciermelt