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A tonic syllable is the most important syllable in a tone unit; it is the syllable that is the main stress of that unit. That being said, an important distinction to understand here is that the tonic syllable, while being the most important stress, is not necessarily the most prominent stress. Tonic syllables and their placements can vary across different languages, but in English there can be only one tonic syllable per tone unit. English tone units may also include a head, pre-head, or a tail, all which reflect different components of the sentence flanking the tonic syllable. 

All this information pertains to a component of linguistics dealing with intonation, or the variation in pitch that expresses a speaker's attitude or feelings, indicates assertion versus questioning, and places emphasis on a particular part of a sentence. Understanding the role of intonation is particularly important for those learning English because our five primary tone types--rise, fall, rise-fall, fall-rise, and level--reinforce emotion, grammar, and rhetoric. It also is a significant factor in controlling the way others listen to us!