According to the dictionary, dominance is defined as power and influence over others. It would then follow that sexual dominance is power and influence over others through the use of sex, but this misses the mark by a wide margin.
This may leave you asking yourself what it is, then. Unfortunately, there is no simple answer. In legal terms, it is forcing someone to do something sexually through threat or administration of physical violence, or the use of psychological, physical, emotional, or sexual abuse to achieve psychological control over another person.
Clinically, it is a sexual deviance wherein the person displaying these traits satisfies psychological drives towards narcissism, sadism, and sexual gratification through the use of coercive tactics over another person. All of these definitions, though technically correct, paint sexual dominance in a very negative light and reflect the fact that there is still so much that is misunderstood about it.
All of that being said, I will explain what I think sexual dominance is. Sexual dominance is exerting control over another person for sexual gratification. However, there is the caveat that the person under control wants to be there. Like all things sexual between people, there is a give and take.
As a sexual dominant, I take pleasure from you by controlling you in some fashion, but you must also take pleasure in being controlled. Otherwise, it is just abuse. At all times, and particularly in any form of sexual activity, it is essential that actions taken are safe, sane, and consensual. Sexual dominance involves the dominant person taking/doing what they want, sexually, while the other participant enjoys the opposite side of the dynamic.
It is entirely possible that this only creates more questions for you, and that is not a bad thing at all. Feel free to ask me anything. I will do my level best to explain. Or, do your own research.
There is a bounty of information on the subject available on the internet, though it may be difficult to find at times. If this is a subject that interests you, explore all facets of it. And, most importantly, have fun with it in a safe, sane, and consensual manner.