This is the 509th most frequent Romanian word.
unii
"Unii" means "some" (referring to people or things) in English.
The word 'unii' is used as an indefinite pronoun referring to an unspecified subset of people.
Unii oameni preferă marea, alții muntele.
Some people prefer the sea, others the mountains.
Here, 'unii' specifies an unknown but specific subset of a defined group ('my students').
Unii dintre elevii mei au participat la olimpiadă.
Some of my students participated in the Olympiad.
In this case, 'unii' introduces a subject group representing an undefined portion of participants.