This is the 40th most frequent Korean word.
대통령은
"The president" (subject/topic marked in Korean).
Here, '대통령은' introduces the subject of the verb 'represent' (대표합니다). The particle '은' indicates that this topic may contrast with or be highlighted against others.
대통령은 국민을 대표합니다.
The president represents the people.
In this sentence, '대통령은' again acts as the subject, focusing on the recent action taken by the president.
대통령은 최근에 새 법안을 발의했습니다.
The president recently proposed a new bill.
This usage demonstrates '대통령은' emphasizing the role or responsibility of the president to make decisions.