This is the 2nd most frequent Japanese word.
の
"の" is a particle used to indicate possession, description, or attribution, often translated as "of" or "'s" in English.
The particle "の" indicates possession, showing ownership between "私" (I) and "本" (book).
私の本です。
It is my book.
The particle "の" indicates a descriptive relationship between "日本" (Japan) and "文化" (culture).
日本の文化が好きです。
I like the culture of Japan.
The particle "の" indicates a location that defines the context of the action, linking "電車" (train) to "中" (inside).