Short answers, pulled from the story.
Planetary migration is an astronomical phenomenon where a planet or other body in orbit around a star interacts with a disk of gas or planetesimals, resulting in the alteration of its orbital parameters. This process changes the semi-major axis and other orbital elements over time.
Type I migration affects small planets through Lindblad and co-rotation resonances that excite spiral density waves in the surrounding gas both interior and exterior to the planet's orbit. In most cases, the outer spiral wave exerts a greater torque than does the inner wave, causing the planet to lose angular momentum and migrate toward the star.
Hot Jupiters are exoplanets with Jovian masses but orbits of only a few days, and their existence contradicts standard theories predicting such planets cannot form so close to stars due to insufficient mass and high temperatures. Planetary migration offers the most likely explanation for these distant-formed giants now orbiting near their host stars after forming while gas remains in protoplanetary disks.
According to the Nice model scenario, Jupiter and Saturn divergently crossed the 2:1 orbital resonance after 500 million years or 600 million years. This crossing increased the eccentricities of both planets and destabilized Uranus and Neptune, leading to encounters where Neptune surged past Uranus and ploughed into the dense planetesimal belt.
Unlike disk migration which lasts only a few million years until gas dissipates, tidal migration continues for billions of years. Tidal evolution produces semi-major axes typically half as large as they were when the gas nebula cleared.