This is the 296th most frequent Korean word.
관한
"Related to" or "about."
In this sentence, '관한' is used to connect the noun '역사' (history) with the noun '책' (book), indicating the topic or subject of the book.
역사에 관한 책을 읽고 있습니다.
I am reading a book about history.
Here, '관한' serves to link '그 사건' (that incident) with '정보' (information), specifying the topic or subject of the information.
그 사건에 관한 정보를 인터넷에서 찾을 수 있습니다.
You can find information about that incident on the internet.
'관한' is used to associate '환경 보호' (environmental protection) with '주제' (theme/subject), indicating what the theme or subject refers to.