This is the 1850th most frequent Russian word.
основным
"Основным" means "main" or "primary" in English.
In this sentence, 'основным' is used as a predicative adjective to describe 'предметом' (subject), explaining that the subject was the primary focus of discussion.
Основным предметом обсуждения был план развития проекта.
The main subject of discussion was the project's development plan.
Here, 'основным' modifies 'приоритетом' (priority), qualifying it as the most important among other priorities.
Он назвал основным приоритетом улучшение качества продукции.
He named the main priority the improvement of product quality.
In the third sentence, 'основным' describes 'фактором' (factor), indicating it as the primary element influencing decisions.