This is the 12th most frequent Chinese word.
大
Big; large.
The word '大' is used here as an adjective to describe the size of the eyes, meaning 'big'.
他有一双大眼睛。
He has a pair of big eyes.
In this sentence, '大' is part of the term '大庭广众', which refers to a public and open area with a large audience or crowd.
小孩子喜欢在大庭广众下表演。
Children like to perform in front of a large audience.
Here, '大' describes the abstract concept of a 'dream', indicating its significance or grandness.