An SVG QR code is a QR code saved in Scalable Vector Graphics format.
Unlike a pixel-based image, the QR pattern in an SVG is described as vector geometry. That means the QR can be resized to very different dimensions without the square modules becoming blurry or pixelated.
W3C defines SVG as a format for two-dimensional vector and mixed vector/raster graphics:
That makes SVG particularly useful when a QR will move between multiple layouts or when you do not yet know the final print size.
With $1 QR Codes, every managed QR can be downloaded as SVG as well as PNG.