Wikipedia is here to provide summaries of accepted knowledge to the public, as described in WP:NOT; generally speaking, the more accepted knowledge it can provide (subject to certain defined limitations on its scope), the better it is. Please boldly add content summarizing accepted knowledge to Wikipedia, either by creating new articles or adding to existing articles, and exercise particular caution when considering removing sourced content. However, it is Wikipedia policy that information in Wikipedia should be verifiable and must not be original research. You are invited to show that content is verifiable by referencing reliable sources. Unsourced content may be challenged and removed, because on Wikipedia a lack of content is better than misleading or false content—Wikipedia's reputation as an encyclopedia depends on the content in articles being verifiable and reliable. To avoid such challenges, the best practice is to provide an "inline citation" at the time the content is added (see: WP:Citing sources for instructions on how to do this, or ask for assistance on the article talk page).
Although reliable sources are required, when developing articles on the basis of sources, avoid copying or closely paraphrasing a copyrighted source. Wikipedia respects others' copyright. You should read the source, understand it, and then express what it says in your own words.
Another way editors can improve an article is by finding a source for existing unsourced content. This is especially true if you come across statements that are potentially controversial. You do not need to be the person who added the content to add a source and citation for it.
Perfection is not required: Wikipedia is a work in progress. Collaborative editing means that incomplete or poorly written first drafts can evolve over time into excellent articles. Even poor articles, if they can be improved, are welcome. For instance, one person may start an article with an overview of a subject or a few random facts. Another may help standardize the article's formatting, or have additional facts and figures or a graphic to add. Yet another may bring better balance to the views represented in the article, and perform fact-checking and sourcing to existing content. At any point during this process, the article may become disorganized or contain substandard writing.
Although perfection is not required, extra care should be taken that articles mentioning living persons (or recently deceased) do not contain contentious content. Contentious content is negative, positive, or is just questionable and is unsourced or poorly sourced. Contentious content should be either verified immediately, with one or more reliable sources and presented in a neutral manner without undue weight, or removed immediately and without waiting for discussion (see the Wikipedia policy biographies of living persons for more on this issue).