The Torah says, "Today, you are all standing together." It describes the entire Jewish people standing firm before HaShem. Everyone, from the most important to the most humble, men, women, and children.
Our sages tell us that the word "today" refers to Rosh Hashanah.
The Jewish people stood before God at that time, just as we stand before Him on Rosh Hashanah, proclaiming Him King of the universe. God said to the people, "I am making a covenant with you today. You are My people and I am your God—for all generations to come.""
Why a covenant? On Rosh Hashanah we feel so close to HaShem - why do we have to promise that we will always feel close to Him and fulfill His will?
God wants the feelings we have on Rosh Hashanah to continue throughout the year, wherever we are and whatever we are doing. This covenant helps make this feeling a part of our lives.
Extracted from Please tell me what the Rebbe says.