Joomla has a feature that let you give a discription to a keyword. As I'm not sure how it works for SEO, is it good or bad to SEO? for example, if it's done by Javascript it's probably hidden for bots (which makes it bad for SEO. Any answer would be much appreciated. Thanks.
Are you sure it lets you give a description for a keyword, or a description for a meta tag? if it's for the meta tags then you will definitely benefit from having a description. I have noticed that some of the keyword boxes for articles don't actually show up in the source code, so it really depends what settings you have. The most important part of SEO for Joomla is installing sh404sef and setting up your search-engine-friendly links. that will help out a LOT
I think you are referring to meta description. Meta tag is part of SEO off page and it's very important. SEF is other more thing, it's for building friendly url for your website so google robot can crawl it better.
If you are new to Joomla it will take you a while to figure out all the SEO tricks. It's not simple and every article does have a place for a description and keywords but it doesn't really do much for your site. As mentioned components like sh404SEF are a good start and then you need to find a plugin to control the SEO for each page, so they don't all have the same description and keywords as your home page. Joomla template makers are now starting to help out with SEO by changing the way the site loads modules too.