Bitmap images, or raster images, are composed of little blocks of color called pixels. Each pixel is assigned a color code and a location, and when mapped together, they form a picture. When you zoom in on a bitmap image, you can see the individual pixels, so there’s a loss of quality.
Vectors are based on geometrical formulas, instead of pixels, to represent images. Because of that, it’s possible to edit it resizing and changing colors without any loss of resolution.
Gravit is mostly a vector editing tool, but basic image editing is also possible.