This page in a nutshell: Readers must be able to check that any of the information within Wikipedia articles is not just made up. This means all material must be attributable to reliable, published sources. Additionally, quotations and any material challenged or likely to be challenged must be supported by inline citations.
In Wikipedia, verifiability means that other people using the encyclopedia can check that the information comes from a reliable source. Wikipedia does not publish original research. Its content is determined by previously published information rather than the beliefs or experiences of its editors. Even if you're sure something is true, it must be verifiable before you can add it. If reliable sources disagree, then maintain a neutral point of view and present what the various sources say, giving each side its due weight.
All material in Wikipedia mainspace, including everything in articles, lists and captions, must be verifiable. All quotations, and any material whose verifiability has been challenged or is likely to be challenged, must include an inline citation that directly supports the material. Any material that needs a source but does not have one may be removed. Please immediately remove contentious material about living people that is unsourced or poorly sourced.
All content must be verifiable. The burden to demonstrate verifiability lies with the editor who adds or restores material, and is satisfied by providing an inline citation to a reliable source that directly supports the contribution.
Attribute all quotations and any material whose verifiability is challenged or likely to be challenged to a reliable, published source using an inline citation. The cited source must clearly support the material as presented in the article. Cite the source clearly and precisely (specifying page, section, or such divisions as may be appropriate). See Citing sources for details of how to do this.
Any material lacking a reliable source directly supporting it may be removed and should not be restored without an inline citation to a reliable source. Whether and how quickly material should be initially removed for not having an inline citation to a reliable source depends on the material and the overall state of the article. In some cases, editors may object if you remove material without giving them time to provide references; consider adding a citation needed tag as an interim step. When tagging or removing material for lacking an inline citation, please state your concern that it may not be possible to find a published reliable source for the content, and therefore it may not be verifiable. If you think the material is verifiable, you are encouraged to provide an inline citation yourself before considering whether to remove or tag it.
Do not leave unsourced or poorly sourced material in an article if it might damage the reputation of living people or existing groups, and do not move it to the talk page. You should also be aware of how the BLP policy applies to groups.