This is the 16048th most frequent Russian word.
работавший
"Работавший" means "having worked" or "someone who worked."
The word 'работавший' is a past participle, indicating a person (the engineer) who previously worked in the company.
Инженер, работавший в нашей компании, придумал новый проект.
The engineer who worked in our company came up with a new project.
Here, 'работавшего' is the past participle form used in the masculine accusative case due to its reference to 'мужчину' (man).
Она встретила мужчину, работавшего когда-то в том же офисе.
She met a man who used to work in the same office.
In this sentence, 'работавшим' is the past participle in the instrumental case, describing the specialist's role with respect to the project.