It is the number of 10-digit (not 12-digit) seconds that have elapsed since the UNIX epoch time less any leap seconds that have occurred since.
In the Terminal, you can generate the currrent time value as:
date +%s
1520823600
and convert it to human readable time format as:
date -j -u -r 1520823600
Mon Mar 12 03:00:00 UTC 2018
or as:
date -j -r 1520823600 "+%FT%T %z"
2018-03-11T23:00:00 -0400
See man date(1), strftime(3).