Short answers, pulled from the story.
KREEP is an acronym built from K (the atomic symbol for potassium), REE (rare-earth elements), and P (phosphorus). It refers to a geochemical component found in some lunar impact breccia and basaltic rocks, notable for its high concentrations of incompatible elements and heat-producing radioactive isotopes including uranium, thorium, and potassium-40.
KREEP formed during the cooling of the lunar magma ocean, which resulted from the high-energy collision between a Mars-sized planet and the early Earth about 4.5 billion years ago. As minerals like olivine, pyroxene, and plagioclase crystallized and separated out, incompatible elements were progressively concentrated into the remaining liquid, ultimately forming a KREEP-rich magma layer sandwiched between the crust and mantle.
KREEP-containing rocks are primarily concentrated underneath the Oceanus Procellarum and the Mare Imbrium, a region now called the Procellarum KREEP Terrane. This was discovered by the gamma-ray spectrometer aboard the Lunar Prospector satellite, which overturned the earlier assumption that KREEP was spread in a uniform layer across the whole Moon.
KREEP contains about one percent by mass of both potassium and phosphorus oxides, 20 to 25 parts per million of rubidium, and lanthanum at concentrations 300 to 350 times those found in carbonaceous chondrites. Most of the potassium, phosphorus, and rare-earth elements are held in the phosphate minerals apatite and merrillite.
KREEP contains potassium and phosphorus, two key components of NPK fertilizer needed for plant growth, as well as uranium and thorium, which are potential fuels for nuclear power generation. The main obstacle to using these resources is that KREEP concentrations are relatively low compared with terrestrial ores, which may make extraction difficult.
The near-side concentration of heat-producing radioactive elements within the crust and mantle of the Procellarum KREEP Terrane is considered the primary reason for the longevity and intensity of mare volcanism on that side of the Moon. Basins far from this terrane, such as Mare Crisium and the South Pole-Aitken basin, show little or no KREEP enrichment even where impacts dug deeply into the crust.