SVG, stands for Scalable Vector Graphics. If you’re using a craft cutter (like a Cricut or Silhouette), these are the most common type of file to download patterns to cut.
They are vector files, meaning that they contain instructions to draw all the lines in your design. You can make this file as big or as small as you want, and the lines will still be smooth. Contrast that with a digital photograph (like a JPG file), which is made up of lots of pixels (squares) of color. If you enlarge it far enough, you’ll see jagged edges. Enlarge it farther and it’s just a bunch of squares. The smooth lines of a vector file are perfect for cutters.