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A proposition is defined to be a statement that can take on exactly of possible truth values: True or False.
Which of these statements is an example of a proposition?
The last statement is not a proposition quite simply because it doesn't assert anything; it asks a question. Thus, there is no truth or falsity in the sentence. Remember this: a proposition must assert something!
The second statement is not a proposition because, if you try to assign it a single truth value, it will also assume the other truth value, and then return to the original truth value, taking you in a loop repeatedly. Thus, you cannot assign it a single truth value. This is actually a famous paradox known as the Liar's Paradox.
This leaves only the first statement. This is indeed a proposition, because it can be given exactly truth value without any contradictions or other issues.
A proposition is true if it is correct, accurate, agreeing with reality, or whichever dictionary definition you prefer. A proposition is false if it is incorrect.
If these definitions weren't very helpful, don't worry. Many logicians are still conflicted over the notions of propositions and truth values, and some say that their definitions are too vague to be useful. However, the intuitive definitions presented here will work just fine.