Stephanus pagination is the system of reference and organisation used in the works of Plato. Plato's Socratic Dialogues, will be divided into numbers - each number covering a few paragraphs. Each number will be divided into equal sections a, b, c, d and e. As such, this system is often used to reference Plato - for example, Symposium 172a would refer the reader to the opening of Plato's Symposium.
This system of pagination is based on an edition of Plato by Henricus Stephanus (Henri Estienne). The numbers refer to page numbers in the various volumes of his edition of 1578. No work spans more than one volume; so, there are not multiple occurrences of the same page number for a single work. As there were multiple volumes, however, the numbers need to be used in conjunction with a title in order to make any sense, i.e. 172a by itself could refer to passages in several dialogues, but Symposium 172a refers only to one passage.
Stephanus numbers are a standardized pagination system intended to make it easier to refer to places in Plato's Dialogs.
In some other translations the stephanus numbers are included in brackets right in the middle of the text.
We will be using stephanus numbers in this course only because there may be some people in class who will be using different translations or different editions of the Dialogs, and this standard pagination can let us all refer to the same spot in the text.