Here rubʿ denotes a young camel born early in the season; it is not the unrelated sense of a quarter, nor the ruined campsite called rabʿ. Radī means perished or dead, and aẓʾār is a plural related to nursing she-camels.
Tajāwub depicts answering calls. The comparison combines acoustic resemblance with a note of grief: it is not simply an assertion that the singer’s voice is pleasant, but an image of repeated, plaintive response.