In Semantics, what is a proposition?

[quocvn]

Hi,
please explain the notion "proposition" used in Semantics. It's very difficult to understand.
If possible, please give me some examples to clarify it.
thank you so much.

[Casiopea]

Does this definition help?

. . . two speakers can say the same thing by uttering different sentences, whether in the same or different languages. For example, when a German speaker utters the sentence ‘Schnee ist weiss’ and an English speaker utters the sentence ‘Snow is white’, they have said the same thing by uttering the sentences they did. Proponents of propositions hold that, speaking strictly, when speakers say the same thing by means of different declarative sentences, there is some (non-linguistic) thing, a proposition, that each has said.

Source: http://plato.stanford.edu/entries/propositions-structured/

The usual convention for naming propositions is to create a noun phrase by prefixing the word that to any sentence which expresses the proposition in question. Thus, that Jones is a bachelor is the proposition expressed by the sentence "Jones is a bachelor".