A proposition is a proposed plan of action, a detailed suggestion. You might consider your friend's suggestion to set up a neighborhood snow-shoveling business after the blizzard a winning proposition. The noun proposition means something presented for consideration.

the act of offering or suggesting something to be considered, accepted, adopted, or done. a plan or scheme proposed. an offer of terms for a transaction, as in business. a thing, matter, or person considered as something to be dealt with or encountered: Keeping diplomatic channels open is a serious proposition.

This kind of sentences are called propositions. If a proposition is true, then we say it has a truth value of "true"; if a proposition is false, its truth value is "false". For example, "Grass is green", and "2 + 5 = 5" are propositions. The first proposition has the truth value of "true" and the second "false".