This is there to compensate for acceleration/deceleration limits of your laser head. The mechanical bits can't instantaneously stop and reverse direction, so it will generate gcode to move (with the laser off) outside the bounds of your engraving area so that the machine will already be up to speed in the correct direction before enabling the laser. Otherwise you would accelerate and decelerate within the bounds of your engraving while the laser was on, leading to the edges of your engraved area having a longer dwell time under the laser and thus getting over-engraved. (It's also possible to achieve something similar by reducing the power relative to the acceleration curves, though I don't know if lightburn supports that.)
I don't specifically know how to configure this in lightburn, but my guess would be it's not necessarily a direct thing you control, but rather it's calculating the necessary over-travel distance needed based on the acceleration parameters of your hardware that are defined in its profile.