This is the 3565th most frequent Russian word.
серьезных
"Serious" (genitive plural or accusative plural of "серьезный").
In this usage, "серьёзных" modifies "разговоров" (conversations), emphasizing their importance or solemn nature.
Он всегда избегал серьёзных разговоров о будущем.
He always avoided serious conversations about the future.
In this sentence, "серьёзных" serves as an adjective modifying "задач" (tasks), indicating their importance or difficulty.
Для решения серьезных задач необходимо проявлять серьёзность.
To solve serious tasks, it is necessary to show seriousness.
Here, "серьёзных" is used as an adjective for "перемен" (changes), signifying their significant impact.