美国 (Měiguó) means "United States" or "America." It is a proper noun composed of 美 (beautiful) and 国 (country), following the Chinese convention of using phonetic-semantic abbreviations for country names. It functions as a noun and can modify other nouns to mean "American."
她在美国留学了四年。
Tā zài Měiguó liúxué le sì nián.
She studied abroad in the United States for four years.
这个产品主要出口到美国。
Zhège chǎnpǐn zhǔyào chūkǒu dào Měiguó.
This product is mainly exported to the United States.
他的朋友是美国人。
Tā de péngyou shì Měiguó rén.
His friend is American.