Bai's poem is just one line long, but it holds a whole night inside it. The speaker is awake at five in the morning, that strange hour when it's too late to count as night and too early to start the day. Instead of sleeping, they are thinking about someone who isn't there, and the thing they miss most is specific: the other person's lips.
The word "still" does a lot of quiet work. It suggests the longing has lasted for hours, maybe much longer, and that the speaker has not managed to shake it off. Nothing is said about why the two are apart, so we are left to guess whether it is a breakup, distance, or something else. That openness lets readers fill in their own story.
The mood is lonely, tender, and a little tired. By choosing something as small and physical as a kiss, the poem makes the ache feel real and close, the way missing someone often does in the quiet before dawn.