Raster images are made of pixels. A pixel is a single point or the smallest single element in a display device. If you zoom in to a raster image you may start to see a lot of little tiny squares.
Vector images are mathematical calculations from one point to another that form lines and shapes. If you zoom into a vector graphic it will always look the same.
A raster image has a specific number of pixels. When you enlarge the image file without changing the number of pixels, the image will look blurry. When you enlarge the file by adding more pixels, the pixels are added randomly throughout the image, rarely producing good results.
When you enlarge a vector graphic, the math formulas stay the same, rendering the same visual graphic no matter the size. Vector graphics can be scaled to any size without losing quality.