Short answers, pulled from the story.
The Late Heavy Bombardment refers to a hypothesized astronomical event where impact rates increased dramatically around 3.9 billion years ago. Radiometric dating of lunar samples collected during Apollo missions in the early 1970s revealed clustering between about 3.8 and 4.1 billion years ago.
The Nice model attributes the Late Heavy Bombardment to dynamical instability in the outer Solar System involving giant planets. Jupiter and Saturn drifted apart until crossing a 2:1 orbital resonance which destabilized Uranus and Neptune onto wider orbits causing comet bombardment.
In 1999, the oldest known rock on Earth dated to 4.031 ± 0.003 billion years old was discovered within the Acasta Gneiss of northwestern Canada. This finding confirmed a boundary across various methods before the LHB hypothesis suggested thousands of craters formed later.
The Planet V hypothesis posits that a fifth terrestrial planet with mass less than half of Mars caused the bombardment when its unstable orbit intersected the inner asteroid belt. Perturbations from other inner planets made this hypothetical planet's orbit unstable causing many asteroids to enter Earth-crossing orbits.
Some scientists argue the cluster of impact melt ages near 3.9 billion years could be an artifact of sampling ejecta from a single basin like Imbrium rather than multiple distinct events. Quantitative modeling indicates substantial amounts of ejecta from the Imbrium event should exist at all Apollo landing locations.