Shortcuts are abbreviations which are used to quickly find policy amd help pages. Such pages tend to have long names, which these prefixes effectively work around. These shortcuts are visible in the upper right hand corner of policy and help pages.
Their nomenclature is pretty straighforward:
T:FIRST_SYLLABLE_OF_FIRST_WORD
Thus, the shortcut for tardis:blocking policy is T:BLOCK, the shortcut for tardis:vandalism policy is T:VAN, and so on. The main exception to this policy is T:MOS, but only because "MOS" is a common abbreviation for the Manual of Style.