"What became of your bloodhounds, Lord Randall my son? What became of your bloodhounds, my handsome young man?"
Lord Randall's mother notices that his dogs (bloodhounds are a kind of hunting dog) are missing. Where did they go?
More nagging questions from the mom. More worries for us readers.
"O they swelled and they died mother, make my bed soon, For I'm weary wi' hunting, and fain wald lie down."
Oh dear… the dogs, it turns out, met a terrible end. They didn't just die—they swelled and they died, which is particularly awful (and by awful, we mean gross). They clearly met an unnatural death.
The refrain swoops in for a fourth go-around. Clearly, the air of menace that has been building with every repetition has come to a head here, with this evidence of tragic dog-murder. We're starting to get the idea that poor Randy is weary from more than just hunting.
Look out next time—the refrain will definitely change in the fifth stanza.