From our documentation: 
When engraving, the head is scanning back and forth. Because the laser has to accelerate and decelerate the head at the beginning and end of each line, on machines with limited power control this can cause darker edges than expected.
Overscanning generates extra moves, past the ends of each line, switching the laser off before it fully stops, or even before it begins to decelerate, allowing the entirety of the engraving to happen at the desired head speed and then decelerating while the laser is not burning. The overscan number is a percentage of your cut speed - the default setting is 2.5%, meaning a cut at 100mm/sec will move an additional 2.5mm past the last cut with the laser off.
Note, Overscan is applied automatically by DSP hardware, like Ruida and Trocen controllers. This setting is only available for gcode based systems which do not do this automatically.
Documentation/Operations.md at master · LightBurnSoftware/Documentation · GitHub