Grey literature is information produced outside of traditional publishing and distribution channels, and can include reports, policy literature, working papers, newsletters, government documents, speeches, white papers, urban plans, and so on.
This information is often produced by organizations "on the ground" (such as government and inter-governmental agencies, non-governmental organisations, and industry) to store information and report on activities, either for their own use or wider sharing and distribution, and without the delays and restrictions of commercial and academic publishing. For that reason, grey literature can be more current than literature in scholarly journals.
However, because grey literature (usually) does not go through a peer review process, the quality can vary a great deal. Be sure to critically evaluate your source.
For an extensive list of the types of documents that might be considered grey literature, see Document types in grey literature (from GreyNet International).
From Simon Fraser University Library: https://www.lib.sfu.ca/help/research-assistance/format-type/grey-literature