An in-system hyperspace jump is a strategy used by many players to escape overwhelming opposition without failing quests thet require you to remain in the system until the quest is complete.
It is a simple strategy. If outnumbered by opponents, all you have to do is to call up the hyperspace screen, click the system thay you're in, and make a jump to another sector in that same system. It is a good diea to shunt the reactor power to the shields while making the jump and waiting for the jump to be calculated, as this furthers your chances of survival.
Depending on the nature of the mission in-system hyperspace jumps may allow you to reach waypoints and targets safely and avoiding enemy fighters. They may also allow you to have repairs made at the space station in the system. Finally, an in-system jump may put some distance between you and the opponents, allowing you to attack them from another angle.
Note, however, that certain targets, such as ships to be boarded, escorted or destroyed will exit the system if you don't reach them in time. An in-system jump placing you further from such a target may jeopardize such missions.