Open an image to find out whether it really contains transparency, and how much of it is see-through.
The checker draws the image to a canvas and reads back the alpha (opacity) channel of every pixel, the ground truth of whether transparency exists, as opposed to trusting the file extension. A PNG saved from a screenshot tool may contain a fully opaque image despite the format supporting transparency, and a white background that merely looks like the page behind a logo is the classic false friend. The report separates fully transparent pixels (invisible), semi-transparent pixels (soft edges, shadows) and opaque pixels, and shows the image over a checkerboard, the standard transparency backdrop, so soft edges are visible.
Formats differ: JPEG has no transparency at all (the checker will always report opaque for a true JPEG), GIF supports only one fully-transparent colour, and PNG/WebP/AVIF support the full alpha range this tool measures. Analysis happens on the device; the image is never uploaded.
My PNG logo shows a white box, is it transparent? If the report says 0% transparent, the logo itself has a white background baked in, it is not the website's fault. Removing a baked-in background needs an editor or a background-removal tool, not a format change.
What are semi-transparent pixels? Pixels between fully visible and fully invisible, used for anti-aliased edges, soft shadows and glows. A logo with clean edges usually has a thin rim of them.
Why does the checkerboard matter? On a white preview, transparent and white are indistinguishable. The checkerboard shows through only where the image is actually transparent.