A raster image is a grid of pixels, and enlarging it makes the pixels show. Vectorizing traces the image into shapes instead. Each region of color becomes a path with a fill, and soft shading can become an SVG gradient. The result is one file that scales to any size.
The script lettering further down is a 12 KB SVG at any size. Enlarged to 4,000 px wide as a PNG, it is 870 KB, and still the same pixels made bigger.
Images made of shapes vectorize best: logos, icons, lettering, flat illustrations and line art. Photos work too, but they turn into many more shapes and a larger file.