Plaiting a dark red love-knot into her long black hair. (line 18)
By this point in the poem, we've probably figured out that Bess and the highwayman are together. Still, this "love-knot" really makes the point. The actual thing would have been a ribbon tied in a knot that would symbolize your love. Bess is braiding the ribbon into her hair. It's also really important that the knot is dark red, a color of passion and romance, but also, of course, blood. Given how things turn out for Bess and the highwayman, it's not a good sign that the poem's main symbol of love is blood red.
But he loved the landlord's daughter, (line 22)
The highwayman isn't the only guy who's into Bess. There's also Tim, the pathetic guy who takes care of the horses. It's not clear exactly how we're supposed to feel about him. Is his love for Bess supposed to be tragic and sweet, or sort of pathetic and disgusting? However you feel, it's pretty clear that we're supposed to compare this kind of secret love to the exciting romance between Bess and the highwayman. Poor Tim doesn't stand a chance, and we think that leads him to rat out the highwayman.
"I'll come to thee by moonlight, though hell should bar the way." (line 30)
The highwayman is definitely courageous, but he also talks a pretty good game. This sounds like just the kind of thing a girl would want to hear from her outlaw boyfriend. Well, at least a girl from 200 years ago. Don't try this line out on your girlfriend now – it would be tough to pull off without the horse and the fancy clothes. In any case, this is probably the clearest statement of true love we get in this poem.