This is the 95th most frequent Russian word.
поэтому
"Therefore" or "that's why."
Connecting clauses showing consequence.
Я хорошо подготовился к экзамену, поэтому я был уверен в своих ответах.
I prepared well for the exam, therefore I was confident in my answers.
Providing an explanation for the second clause.
У него было много работы, поэтому он не смог прийти на встречу.
He had a lot of work, therefore he couldn't attend the meeting.
Linking the reason to an action taken based on it.