The Fuzz setting defaults to 5%. This controls how strong the transparency is. By that I mean how it treats edges especially where you have shading like on this sample image I'm using.

The transparent background appears as squares but you can see from the image below that the shadow needs to be fixed.

Change the Fuzz to 15% and select anywhere on the background. You will get an enhanced image like below with the shading gone.

On the next page we save as PNG and use it to create a professional image for our website or email campaign.