Wikipedia, as an online user-edited encyclopedia, contains information on just about any subject one could wish to learn, at least in terms of information. It isn't a good place to learn how to do things. However, because it is user-edited, the authority of information on Wikipedia is not always reliable.
Wikipedia has millions of articles covering thousands of subjects. A Simple English Wikipedia also exists to break down hard-to-understand content such as advanced mathematics or science so readers can digest the information. While it can be useful for getting a starting grasp on a topic, using Wikipedia as a primary research source is ill-advised in many contexts, especially academic ones. In fact, many universities refuse to allow Wikipedia as a cited source in research papers.
At the end of each Wikipedia article, a list of sources cited in the article is available. It's best to verify information in these sources because some of them go more in-depth about the relevant material than the article and can provide a more knowledgeable and unbiased viewpoint.
Wikipedia, like most wikis, includes hyperlinks to related articles within the text, which makes it easy to go off on unrelated tangential reading across a wide array of topics.