as for where:
(a reference would have been good, but others not willing to spend 30 secs to look either)
try the PICAXE Manual 2 pages 44, 49, 77, 78, 80, 81, 132 and 150.
Each example programinvolved has a main program part with the line:
gosub flsh ‘ call sub-procedure
then further down the label
flsh:
It is just a name used as a reference for the GOSUB command
flsh: was used because the subroutine is flashing an output on and off once per call.
its an abbreviation for flash and in fact, for easier reading, you could use a label like:
Flash_OP, ; OP for output
or
FLash_LED
in every case where flsh exists.