For graphics to be scalable means they must be able to increase or decrease uniformly. Being scalable means not being limited to a single, fixed, pixel size. SVG graphics are scalable to different display resolutions. The same SVG graphic can be placed at different sizes to the same application, and re-used at different sizes on different applications. SVG graphics can also be magnified to reveal fine detail up close.
Advantages that (scalable) vector graphics have over rasters:
Adaptability:
Content can adapt to any screen size and display properties
Same content can be displayed on desktop, handheld, phone, etc.
Zooming capabilities:
Content can be zoomed with no loss of quality or need to reload
Animation:
Content can be animated in a variety of ways (not just frame based)
Interactivity:
Content can be interactive (for example, Flash)
Searchability:
Content can be searched for text or shapes
Efficiency:
Content can be compressed using gzip (standard)
Animation compresses ten times better than animated GIF