Stumbleupon was a way to collect and discover random good things on the web before places like Pinterest. It was basically a time-killer. You'd install the toolbar control, and if you wanted to, you hit the "stumble" button and it would take you to somewhere another Stumbleupon user had recommended as being worth seeing. As a user, you can customize your interests so that when you stumble, you're likely to see things of interest to you.
As a submitter, you're providing links for other people to your content. If you add stuff to Stumbleupon, it usually presents you with a short form to fill out that gives you the opportunity to choose a category, keywords, and to provide a description. If you go to one of your images and click the green "SU" button on the share links, you'll see that form and can add your image to the Stumbleupon database so that it will possibly appear in front of someone who is "stumbling" and is interested in one of your keywords or categories.
