I was told by Adsense Team that my website contains copyrighted materials. I have no idea what it is...

1. Is there any content on your site that is not your original creation?

2. Is there any content on your site that was created by someone else and you didn't properly cite the author/owner?

If you answered Yes to either of the above, that content either is your current problem or will likely be a problem in the future. If you want to keep your AdSense account and/or your Google search ranking, get rid of that content or cite properly.

I know Wikipedia content is free to use, but I'm not quite sure what kind of label or statement I should put under the contents.

You might read this, especially the part about contacting the original author for permission - [en.wikipedia.org...]

How to cite wikipedia content legally correctly?

There are some standardized methods for citing someone else's work. If you can't recall from high school English composition class, try a Google search, there's lots of information available.

Note: To "cite" something is not the same as just copying it and pasting it on your web site.

FarmBoy

Wikipedia content is not the "source"...you have to cite from the source. Follow the works cited at the end of the article back to the original links.

wikipedia should never be a source.
Take care with wikipedia sources though: I know of instances where they kicked out for some impossible to understand reason the original references and replaced them with other references.

As for "website contains copyrighted materials": wikipedia content is copyrighted (and the stuff they quote as well in all likelyhood), just referencing might or might not get you "fair use", but permission will solve it properly.

Text on Wikipedia is under the GFDL license. Term for using the text are here:

[en.wikipedia.org...]