Those two storage format types describe how each data fragment participates on unpacking and rendering the JPEG file.

The browser can start rendering parts of the JPEG immediately as data arrives but only after the browser read the header of the JPEG. In the header is either baseline or progressive defined.

Progressive

As we can see with each chunk of data that arrives the quality increases. (I used a gausian blur just for illustration). The size of the picture never changes; only more details pop in.

Baseline

Here we can see that line by line the picture is assembled by the browser until all data fragments arrive. The size (height) of the picture changes with each data fragment. The quality of each rendered pixel line is from the beginning as good as it is at the end of the process.