A "Widget" is, in simple terms, a block of code or text that can be easily manipulated in a hub (known as "capsules") or blog (typically WordPress uses widgets for ease of use).
So, rather than hand-coding CSS or PHP, HTML or what have you, there is a basic framework (the widget) that allows a new user or someone trying to save time to simply type in their content, and move the widget to the spot they want it displayed.
It's a general definition, and it really depends on what you're talking about, but that's the general idea. Hope that helps.
The RSS feed is "Really Simple Syndication," there is another more technical name for it but the idea is the same. It is the internet's version of a newspaper for your hubs/blogs/website.
Say I had wanted to follow your new hubs, Samuel. I could either go to your hubs and check every day, or I could grab your RSS feed and plug it into my reader. Personally I use Google's reader.
On my Google homepage, I have these "newspapers" from all different sources, people whose blogs and hubs I enjoy reading. For me, I put these either on my iGoogle homepage and have a widget of the RSS feeds displaying titles of recent hubs/entries, or I'll put them in my Google reader and get to them when I can.
It's a great way to get more readership, especially if you publicize your hubs/feeds. To do that, I use RSSBot from Incansoft (I think it's $15), Magic Submitter, and sometimes I'll use KSoft's Feed Submit (it's the biggest RSS feed submitter I know of, it has about 80 or so RSS directories and can plug into a particular website that has published 260+ RSS directories).
Hope that helps. Oh, for the record, you can find or create an RSS feed from any web property. Most, like HubPages, provide one for you.