This is the 8581st most frequent Russian word.
один
"One."
In this sentence, 'один' is used as a numeral meaning 'one', specifying the quantity of the object being referred to.
Я хочу один бутерброд.
I want one sandwich.
Here, 'одна' is the feminine form of 'один' and is used to mean 'alone', indicating isolation or singular presence.
Она живёт в квартире одна.
She lives in the apartment alone.
In this context, 'один' is used within a phrase 'один из' meaning 'one of', introducing a part of a set.