Instead of a collection of pixels which comprise a raster image, vector images are made up of sets of coordinates that represent shapes with color fills assigned.
Because they are a description of how to create a raster image, rather than an actual raster themselves, true vector graphics can be rendered at any desired resolution with no loss in quality.
Note that Twibooru generates PNG previews of SVG files which are presented at their nominal size, but keeps the original SVG versions available for download.

If the artist has made a SVG or AI file available at the source, please add the appropriate tag: .svg available or .ai available.
Better yet, upload that original file.

Additionally, please add the tag transparent background if the background of the image is partly or wholly transparent (very common on vector images).