Keyword stuffing is where you just simply mention the word or phrase too many times on the page. If you read the text yourself and it sounds spammy because the same word or phrase is used over and over again then you are keyword stuffing.
Remember keyword stuffing can also be due to the use of the ALT tag on pictures. To see what your alt tags contain open a page using Firefox, right hand click on an image, select 'View Image Info' and then see what is written next to the 'Associated text' label. Is it descriptive or spammy?
Hidden text is where you, or your SEO company, adds text to a page that is the same color as the background in order to give search engine crawlers that there is more content/different content on the page than there actually is or that the content of the page is about something that it isn't!
If you want to check for hidden text just open any page with Firefox that Google has highlighted as having an issue, right hand click on an empty space and click 'Select All' which will highlight all the text on the page, visible or not.
For keyword stuffing:
For hidden text:
I'm Tim Hill, a Search Engine Optimization and Online Marketing specialist. I created this site to help others understand that SEO is not a mysterious black art!.
If your a newbie try the Getting Started in SEO page, otherwise feel free to dig around and learn more.