Someone once told me: if Wikipedia is a battle, citations are the bullets. Well, Wikipedia isn't a battleground (not most of the time) but citations still matter. And just like math class, you need to show your work.
No doubt, a lot of Wikipedia's information lacks sources, most without being flagged "citation needed". In practice, citing sources is strongly encouraged, but only required when information might be challenged. But that could really be any time, so including citations is always best.
To cite a source, the very least you need to do is link to where the information came from.
If you're editing the "wikicode", put tags around your link like this: <ref>LINK</ref>. If you're using the VisualEditor, click the "Cite" button at the top of the page, enter a link for it to be automatically detected, or write it up yourself. There's more to it, of course, but that's good for now!