Get out the microscope, because we’re going through this poem line-by-line.
"One kiss, my bonny sweetheart, I'm after a prize tonight,
But I shall be back with the yellow gold before the morning light;
- Now it's back to the highwayman, who tells his "bonny [pretty] sweetheart" that he's headed off to do some robbing tonight.
- He promises her that he'll be back by morning, with gold in hand.
Yet, if they press me sharply, and harry me through the day,
Then look for me by moonlight,
Watch for me by moonlight,
I'll come to thee by moonlight, though hell should bar the way."
- But…(and this is important) the highwayman warns Bess that he might be chased, that the law might "harry" (bother and harass) him all day.
- In that case, he tells her to wait for nighttime ("moonlight"). Then he promises he'll come see her, no matter what, even if "hell should bar the way" (we might say "come hell or high water").