Short answers, pulled from the story.
A sednoid is a trans-Neptunian object with a large semi-major axis, a distant perihelion beyond 60 AU, and a highly eccentric orbit similar to that of the dwarf planet Sedna. Only four objects are generally agreed to belong to this class: Sedna, Biden, 541132 Leleākūhonua, and Ammonite.
Four sednoids are currently recognized by consensus: Sedna (discovered 2003), Biden (2012), 541132 Leleākūhonua (announced the 1st of October 2018), and Ammonite (2023). All four have perihelia greater than 60 AU.
The orbits of sednoids cannot be accounted for by the gravity of the giant planets or by galactic tides. Competing hypotheses include a stellar flyby during the Sun's birth cluster phase, gravitational capture from passing stars, disruption by a hypothetical Planet Nine, or influence from a rogue planet that passed through the early solar system.
The first three known sednoids share an unusually similar orbital orientation that should have been randomized over billions of years. One explanation is a hypothetical super-Earth at roughly 250 AU, dubbed Planet Nine, which could hold these objects in a stable alignment for billions of years. Such a body would be too faint for current all-sky surveys to detect.
In 2006, Brown called Sedna a fossil record of the earliest solar system, saying that when other such objects are found, Sedna will help reveal how the Sun formed and how many stars were close to it at formation. A survey Brown conducted with Rabinowitz and Schwamb in 2007-2008 found no new sednoids but suggested about 40 Sedna-sized objects likely exist in the same region.
Following the discovery of Leleākūhonua, Sheppard and colleagues estimated that roughly 2 million Inner Oort Cloud objects larger than 40 kilometers exist, with a combined mass roughly equal to the mass of Pluto and several times the mass of the asteroid belt.