how to understand seconds_behind_master

[jiangwen jiang]

Hi, all:

I am confusing with seconds_behind_master, in mysql manual, it says:
In essence, this field measures the time difference in seconds between the
slave SQL thread and the slave I/O thread.

how to understand this description:
1. seconds_behinds_master = ts in SQL thread - ts in I/O thread, or
    seconds_behinds_mater = ts in I/O thread - ts in SQL thead
2. how to get the timestamp in SQL and I/O thread, when this time will
update?

Thanks
W.J