From that sentence: the autumn wind is slow, the street lights are a little bit, the couple are both shadows, and the sea is vast.
The first two sentences of this sentence describe the surrounding scenery. At night, the autumn wind is slow, and autumn is full of depression, which also renders people's bleak mood. Street lamp is a description of the lively scene, comparing the bustling market with the surrounding pairs of people, and then thinking that you are alone, you can't help but sigh.