$32=0 is “spindle mode”. It means that the controller will pause each time the PWM output is changed, because it assumes you have a rotary tool (spindle) that needs time to speed up / slow down to the requested RPM. It also will not disable the PWM output when executing travel moves because it assumes the spindle is retracted.
$32=1 is “laser mode”. It eliminates the pauses when the PWM output is changed because lasers respond to this instantly, and it disables the PWM output (IE the beam) during travel moves.
Note that this requires GRBL 1.1f or later firmware. If your GRBL version is older than 1.1f, you would need to use the GRBL-M3 device in LightBurn, which emits different GCode that manages the beam output directly, instead of assuming the GRBL will turn it off/on for you during travel / cut moves.