If you want people visiting your portfolio to be able to download and print (or steal) your image in its full original resolution, then that is what you would upload. In this case (which I think most people do NOT want) you simply un-check the "Resize to Fit" option - which disables that whole section apart from Resolution. For screen viewing it doesn't matter what PPI number goes into the Resolution box - it makes no practical difference. Most gallery websites will in this case themselves generate a scaled-to-fit image version on the fly, as needed.
If you want to upload a downsampled image - in other words, something which is perfectly good for viewing the whole photo on screen, but which does not release the entire fine detail of it onto the internet - then you will need to check the "Resize to Fit" option.
Now you need to define the number of pixels wide and high, that are to be produced. This can be done the complex way or the easy way. The complex way is by deciding on a size in inches (or cm) and then guessing what the PPI is going to be of somebody else's monitor, and typing that in as your Resolution, and that results in a certain number of pixels. The easy way is to type in a certain number of pixels directly and simply disregard the PPI. I usually type in 1680 px wide x 1200 px high (landscape orientation images can display larger than portrait ones can, since peoples' screens are generally landscape).
Many people are familiar with the "complex" method of assigning sizing to digital images, from printing - which is a situation where you ARE in control of the pixels-per-inch, and where at a given size, PPI has a loosely-speaking "quality" relevance. But for an on-screen gallery, all you can affect are the pixel dimensions (so the "easy" method makes more sense IMO) - the PPI that is seen by the viewer, is completely outside your control.