The command creates the reap.sec file from the reap.list file. The reap.list file is a text file with 2 parts separated by character comma "," in a line. The upper line must be more recent.
The first part is epoch when an additional second was given in you system local time, which using "date" command. The format of the epoch is the same as the output of "date" command, which consists 5 parts separated by character space " "; three letter codes of the day in a week and of the month, date in the month, local standard time, chasoric year. For example
Fri Jan 1 09:00:00 1999
The second part is an integer which is equal to the integration of the additional seconds, or the difference between DT (the Dynamical Time, celestial mechanicians are used) and UTC. DT was called ET (the Ephemeris Time) in early days.
The line started by character "#" is comments and the command skips this line. And the last line should be written only the word "end".
The target file "reap.sec" is a text file with two columns separated by character space " ". The first column is expire epoch which is time_t integer. The second column is the additional second integrated over.