Short answers, pulled from the story.
The retreat of glaciers since 1850 is the well documented worldwide shrinking of mountain glaciers, tracked with staked terminus markers, GPS surveys and satellite altimetry. Between 1993 and 2018 alone, glaciers lost roughly 5,500 gigatons of ice, or about 210 gigatons a year.
Glaciers have been retreating because their mass balance, the difference between snow gained in the accumulation zone and ice lost in the ablation zone, has turned negative almost everywhere. Human driven warming since the start of the industrial era, especially the sharp acceleration after 1980, is the principal driver.
Glacier retreat since 1850 has been fastest since 1980, after a period between 1950 and 1980 when retreat slowed and, in some cases, reversed as global temperatures cooled slightly. Before that, glaciers retreated steadily from the end of the Little Ice Age around 1850 until about 1940.
The retreat of glaciers since 1850 raises sea levels directly through meltwater runoff, an effect the IPCC calls a slow onset event. Greenland's ice sheet loss roughly doubled from about 90 cubic kilometers a year in 1996 to 220 cubic kilometers a year by 2005, while a 2019 study found Antarctica losing ice six times faster than 40 years earlier.
The retreat of glaciers since 1850 threatens water supplies for large downstream populations, including a cut of as much as two thirds in summer water flows in the Himalayas. That would leave roughly 500 million people in the Ganges basin and 1.4 billion people across the wider Hindu Kush Himalaya region short of water.
Glacier retreat since 1850 has produced glacial lake outburst floods, or GLOFs, when meltwater trapped behind unstable moraines breaks free. In 1892, a GLOF from the Tete Rousse Glacier killed 200 people in Saint-Gervais-les-Bains, France, and in 1994 a GLOF from Bhutan's Luggye Tsho lake killed 23 people.