Every object in a star system has mass and exerts a gravity well on its region of space time. So the star (or stars in a binary or more star system) exerts a gravitational force on space time and causes a curving of space time or also known as a distortion.
https://blog.edecker.net/wp-content/upl ... ells-2.png
Imagine a black blanket on your bed and you put heavy objects on that blanketed bed. Each heavy object makes the mattress sink in proportion to the mass each contains.
The too often used slingshot effect to increase acceleration around a star seen in many science fiction films like 2010 or Deep Impact is a method proposed to increase a speed and acceleration of a space vessel so it exceeds the possible thrust it can induce itself...by traversing a gravity well across the curvature of space time.
It's not completely accurate as it's limited to one plane (the geometry use of the word), but imagine you are a miniature human standing on a billiard table except add 100 balls on it. Now your task is to dash through in the shortest possible distance and varying speed and acceleration without getting crushed by the moving balls. Good luck as they are not moving at a uniform speed and acceleration.
So imagine doing this for all the stars as seen in the first image in all four axis and planets and moons too. That is just one sector. Most of space time is empty, but still when traversing, it's highly complex, and the smallest error in navigation would have you smash into one of those objects.
And it's not just that, but caught in gravity wells. And you might skip across the outer atmosphere of a planet like a stone across the surface tension of a pond...but causing a firey release of heat due to friction. Or you might get caught in a solar flare emmision which are random but ordinarily have ranges, though an unsuspected burstcan extend as far as the sun to the Earth and cause shearing forces on the starship.
