The Portable Network Graphics (PNG) file format allows transparency in your image to be stored by saving an alpha channel. You can uncheck the box if you are not using transparency and you want to make the resulting file smaller. Always saving the alpha channel is recommended.
O formato de ficheiro 'Portable Network Graphics', PNG, permite gardar as transparencias pois gárdaas na canle alfa. Pode desmarcar esta caixiña se non quere empregar transparencias e se quere facer que o ficheiro resultante sexa máis pequeno. Recoméndase sempre gardar a canle alfa.