See Serpent for an explanation of the symbolic meaning.
In Earth's Galactic History, it is interesting to know that the physical Sirians were called the snake-people, and used a serpent as a symbol for their people.
They were however not the only ones to carry this name and symbol: the Lemurian record keepers too were called the snake people, as are some tribes of the native Americans. And that, of course, is how the lineage went: Sirians, interacting with Lemurians, who then fled first fled to South, and later to North America.
Sometimes, however, the term "snake-people" is also used to refer to the reptilian races.