Edge burns or flares

You see

A fill is uniform in the middle but has darker spots at the start and end of each line. Or a cut has heavier char where the laser turned on / off.

When the head accelerates from a stop or decelerates to a stop, it moves slowly for a moment. If the laser is firing during those moments at full power, the slow head deposits extra energy in a small area, burning.

Fix

If you are engraving fills

This is what overscan prevents. With overscan, the head reaches full speed before firing and continues past the line end before stopping. The fire-and-decelerate motion happens outside the fill boundary.

Overscan is added automatically by the planner. If you still see edge burns, the cause is usually one of:

$32=1 (laser mode) is off in the firmware. Motion-synchronized power requires both $32=1 and dynamic M4 output. Set $32=1 via the Console. See GRBL configuration.
Output Policy is wrong. In Device Settings → Profiles → Output Policy, make sure Constant Power (M3) is off so the profile emits dynamic M4. M3 remains constant-power output even when $32=1 and can cause edge burns.

If you are cutting (Line mode)

Edge burns at start / end of cut paths usually mean the laser stays on too long at the start or end. Some options:

The Optimization Settings modal (Laser Control → Optimization...) has a "Reduce direction changes" option that can help if there are many small cuts that all start at the same point.
For cuts where the visible char at start would be a problem, use tabs so the start point is on the tab (less visible).

If you see darker spots in the middle of fill lines, too

Different problem, usually mechanical. See Visible banding.

Verify it worked

Fill lines are uniform from start to end.
Cut starts and ends show clean breaks without extra char.

Still stuck?

Verify $32=1 is set and Output Policy emits dynamic M4.
Verify Constant Power (M3) is off.
Overscan explainer.
GRBL essentials.
Post in the Facebook group with photos of the edge burns.