In the virtual preview for your canvas print, you’ll be asked to choose the type of wrap. “No wrap” (or no bleed) means your canvas will have white edges. If you choose “Wrapped” (with bleed) your photo will be printed to the edge of the canvas but you’ll lose 40mm of your image all around. “Mirror wrap” lets you print to the edge but without losing any of your image, by copying the outer border of your photo and extending it to the edges of the canvas.
You’ll be able to see exactly how each option affects your image on the virtual preview.