Nearly all image compression method has a technique for browsers to display an approximate image while the file is loading.
GIF calls it interlaced: when this bit in the Image Descriptor is set, there are four sub-images coded after each other and each sub-image contains only a portion of the final image: every eighth, fourth, etc. lines.
Without interlace the image would look like this:
Copyright whomever has it
This type of storage makes it possible to display the image progressively:
The GIF-decoder in RAINBOW Library decodes the whole image as if non-interlaced.