Because the radius of the orbits of the planets, our view of our celestial neighbors will sometimes catch them in a direct point-to-point linier view. But the planets don't stop, but because we are curving along the eclipbical orbit, our view of (in this case) Saturn is extended, but that extension also causes the body to move "backwards" in the sky thus the Retrograde label.
It was the irregular obit that the denziens of the pre-columbian 'flat=Earth' that thought that these lights in the sky that did their own thing were Gods, hence why the planets were named for the ancient Pantheon.
When a body goes retrograde, its energies get redirected to other goals. Saturn in Cancer puts stress and reality on family scenerios, diffficult changes are being accepted, and denied at the same time. Cancers and Capricorns will be the most affected, Leos will feel Saturn backing off spurring leadership qualities. Jupiter in Libra brings forth fortunne unrestricted by Saturn's move backwards.
Mercury will be going retrograde in December in Sagitatrius and bring some coummunication and travel concerns..