We tend to think of websites as constructed of three elements. Firstly there is the content itself – the words, the images, the links that join it altogether, headings, lists etc. This is written in HTML which does not in and of itself have any information about what it all looks like. The second part is the look and feel of the site, font-faces, sizes, what sits where, colours, background, borders etc. This is handled by the stylesheet(s) written in CSS. Finally there is interactivity which is handled by scripting (programmes) either at the server end (where the website code sits) or at the client (web browser) end.
From our point of view these MAY enable us to do some things (like forms for example) in a more user-friendly way, without a large cost in terms of development time. The downside is that for most libraries the user has to download the entire library, so there is more investigation to be done here. Simon was in particular introducing a library called jQuery. He liked it because its well documented, easy to learn and has a good balance between simplicity and productivity – sounds good to me.