This is the 17686th most frequent Russian word.
уголовным
"Уголовным" means "criminal" (relating to crime or criminal law).
In this sentence, 'уголовным' is used as an adjective describing 'расследованием' (investigation), indicating that it relates to crimes or criminal matters.
Он занимался уголовным расследованием.
He was engaged in a criminal investigation.
Here, 'уголовным' modifies 'дело' (case), specifying that it pertains to crime or law violation.
Дело оказалось уголовным, так как были нарушены законы.
The case turned out to be criminal because laws were violated.
In this context, 'уголовным' describes 'миром' (world), referring to the realm or environment associated with crimes and criminals.