What is the Tropic of Capricorn and Tropic of Cancer? Why are they so important?
The importance is with their latitudes 23.4 deg S and 23.4 deg N. This angle 23.4 deg is the angle which the axis of rotation of the Earth's spin makes with the normal to the ecliptic.
The spin-axis terminals North Pole ( latitude 90 deg N ) and South Pole ( latitude 90 deg S ) execute 'Simple Harmonic Motion, relative to the Sun, with period 1 year', in the respective line joining the Sun.
Accordingly, the line of centers of the Earth and the Sun turns up and down, in-between Tropic of Cancer and Tropic of Capricorn.