This is the 186th most frequent Polish word.
dlatego
"Therefore" or "that's why."
"Dlatego" is used as a conjunction here to connect the cause (being tired) with its effect (going to sleep earlier).
Byłem zmęczony, dlatego poszedłem spać wcześniej.
I was tired, therefore I went to sleep earlier.
"Dlatego" is used to establish a causal relationship between James liking sports and his daily visits to the gym.
James lubi sport, dlatego codziennie chodzi na siłownię.
James likes sports, therefore he goes to the gym every day.
"Dlatego" indicates the reason for Anna's absence linked to her lack of knowledge about the exam date.