What is the {_ _ ob__: Observer} object of vue? What's the function?

when the data obtained by the parent component asynchronously is passed to the child component through props, I print the passed value and print the following object. Although the transferred data is already displayed on the web page, there is really nothing in the printed object. Why? Ask for
export default {
name: "cityAlphabet",
props: {

cities: Object,

},
mounted () {

console.log(this.cities);

}
}
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May.22,2021
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