He rose upright in the stirrups; he scarce could reach her hand, But she loosened her hair in the casement. His face burnt like a brand As the black cascade of perfume came tumbling over his breast;
Now we hear about the kiss goodbye, which is a pretty romantic moment in the poem.
The highwayman stands up in his saddle, and reaches up (Bess must be leaning out of a high window), just barely grabbing her hand. Then she lets her hair down from the window (the casement), a little like Rapunzel. He blushes bright red, like hot iron ("a brand") when her sweet smelling hair tumbles over him.
This is definitely supposed to be a sexy scene.
And he kissed its waves in the moonlight, (Oh, sweet black waves in the moonlight!) Then he tugged at his rein in the moonlight, and galloped away to the West.
The highwayman kisses Bess's hair, and the speaker makes a big deal about its "sweet black waves."
Notice that he also mentions the moonlight three times in three lines – it's a big part of the atmosphere of this poem.
Then the highwayman grabs the reins of his horse and takes off to do some robbing. That ends the first scene in this poem.