This is the 64th most frequent Japanese word.
約
"約" means "approximately," "about," or "promise."
In this sentence, "約" (yaku) is used as an adverb to express approximation or 'about'. It shows that the duration of waiting was approximately two hours.
私は約2時間待ちました。
I waited for about two hours.
In this sentence, "約" is part of the word "約束" (yakusoku, promise), where it contributes the meaning of binding or agreement within the compound.
彼と約束しました。
I made a promise with him.
Here, "約" (yaku) is functioning as an adverb to indicate an approximate quantity of people involved in the project.