This is the 10673rd most frequent Russian word.
волонтерским
"Volunteer" (adjective, relating to voluntary or volunteer work).
The word "волонтерском" here is an adjective in instrumental case, describing 'movement.' It signifies that the movement is related to volunteering.
Он участвовал в волонтерском движении и помогал организовывать мероприятия.
He participated in the volunteer movement and helped organize events.
The word "волонтерским" is used in instrumental case as an adjective, referring to the nature of the project, indicating its voluntary basis.
Эта программа является волонтерским проектом, направленным на помощь детям.
This program is a volunteer project aimed at helping children.
Here, "волонтерском" describes 'initiative' in the prepositional case, indicating the voluntary nature associated with the initiative.