This is the 145th most frequent Russian word.
дело
"Дело" - matter, business, affair, or case.
In this sentence, 'дело' is used to mean 'matter' or 'issue', referring to a pressing or important topic or situation.
Это серьёзное дело.
This is a serious matter.
Here, 'дело' refers to 'work', 'profession', or 'business', indicating an occupation or task.
Он занялся своим делом.
He engaged in his own work/profession.
In this sentence, 'дела' (plural of 'дело') is used to mean 'business' or 'tasks', referring to activities or matters that need attention.