Each statement will have the ability to have sources. This is not currently supported by the UI (hence everyone is being a bit tentative and only putting in really obvious, uncontentious things) but when it does, it'll basically contain expressions of the form "X has property Y with a value of Z (type T), according to sources A, B and C".
Similar to Wikipedia. That is, every triple ("statement") present in the Wikidata may have an associated source. This is a mechanized version of [citation needed]...
