It is related to foot worship in a derivative way, in that the adulation may really be attributable to the proximity of the boots to their master/mistress.
This reverence for the footwear that encloses the dominant partner's foot is sometimes an expression of extreme devotion or loyalty, sometimes a concrete admission of inferiority or defeat, and sometimes both.
It may also consist, more passively, of the submissive lying prone and being "trampled" by the dominant partner.
Heeled boots may lend themselves to sucking, in an approximation of fellatio, anal and urethra insertion.
Bootlicking also has a political connotation of being a sycophant or having "deference to power undertaken entirely for its own sake, rather than out of fear or even material gain.
Although not much pain is induced in this situation compared to trampling, the slave could be psychologically (and willingly) abused to a large extent.