JPG uses lossy compression and is well suited to photographs, gradients, and colorful web images, often preserving acceptable appearance at a compact size. It has no transparency and is less effective for pixel-sharp text, interface lines, and repeated geometry.
The converter decodes the source in the browser and encodes a new JPG with an adjustable quality setting. This is not an extension rename: transparent pixels need an opaque background, while animation and some metadata are not preserved.