what is sire
Sire is a respectful form of address for reigning kings in Europe. It has been used in Belgium, France, Italy, Germany, Portugal, Sweden, Spain and the United Kingdom. Historically Sire had a wider usage. During the Middle Ages, Sire was generally used to address a superior, a person of importance or in a position of authority or the nobility in general. The words sire and sir, as well as the French (mon)sieur, share a common etymological origin, all ultimately being related to the Latin senior.
Very simply; a Sire is the father (the whole male of any species, equine or otherwise can sire offspring) and the Dam is the mother of the horse. Then, variations on those are like variations on human hierarchies; such as grandsire.