Translation:Our son is in the US.
们 is used to pluralize personal pronouns e.g. 我们， 他们， 你们。 You will often hear that Chinese only pluralize pronouns however this is not entirely true as people nouns are often pluralized using 们 eg. Son - 儿子 sons - 儿子们 child - 孩子 children - 孩子们. To the best of my knowledge objects are never pluralized.