This is the 7603rd most frequent German word.
großteils
"Großteils" means "for the most part" or "mostly" in English.
In this sentence, 'größtenteils' indicates that the seminar was overall interesting, though there might have been some less interesting parts.
Das Seminar war größtenteils interessant.
The seminar was mostly interesting.
Here, 'größtenteils' conveys that most of the objectives of the goal were completed, albeit perhaps not all.
Das Ziel wurde größtenteils erreicht.
The goal was mostly achieved.
In this sentence, 'größtenteils' specifies that the primary impact of the change is on new employees, with some possible minor effects on others as well.