"Mesoret" means "tradition," but it also alludes to the "Masora," the scholarly discipline that concerned itself with the precise transmission of each word and letter of the Bible.
Literally translated, the title can thus be rendered "The Tradition of the Six Orders [of the Talmud]."
It consists of cross-references to other passages in the Talmud where the same quote appears.
Depending on how much space is available on the page, the link from the Talmud text can be designated by an asterisk (*) that points to a reference in the nearby margin; or by a Hebrew letter pointing to a list of references that are assembled together in the margin or at the bottom of the page.
In later printings of the Talmud, additional references (and other notes) were added to the "Mesoret Ha-Sha"S," most of them by Rabbi Isaiah Berlin. These are enclosed in square brackets.