This is the 385th most frequent Polish word.
jeden
"Jeden" means "one" in English.
In this sentence, "jeden" is used as a numeral meaning "one" to specify the quantity of the noun "długopis" (pen).
Mam tylko jeden długopis.
I have only one pen.
In this sentence, "jeden" emphasizes the inclusion of every single individual, used for emphasis.
Każdy jeden chciał zobaczyć ten film.
Every single person wanted to see this movie.
Here, "jeden" is used to indicate one item or person out of a group, in this case one friend out of a group of friends.