A proposition is an assertion that something is the case. We use sentences to express propositions. A sentence may be made of black ink, be on a page, and be four inches long. But the content of the sentence cannot be found on the page. The proposition it expresses appears to be a non-physical entity which can be in the mind.
A proposition can have the following truth values: true or false. "P" is true if and only if P. "P" is false if and only if not P. In other words, a proposition is true if and only if what it says about the world is in fact the way the world is. Though this correspondence view of truth has long been questioned, and especially so in these postmodern times, it remains our common sense, everyday understanding of truth.