multiple process of PostgreSQL running on the same port and same directory

This is the processes I catched:

postgres 5260 0.0 0.0 1224332 37232 ? S Jul13 0:13 \
/opt/pg94/bin/postgres -D /export/pg940_data

postgres 31193 0.0 0.0 1224332 976 ? Ss 00:20 0:00 \
/opt/pg94/bin/postgres -D /export/pg940_data

I found the similar question: ps output shows multiple instances of
PostgreSQL running on the same port
<http://www.postgresql.org/message-id/27699.1347932194@sss.pgh.pa.us> ,
but I wanna konw when to fork a postmaster process, and what does it do.

PostgreSQL version:
postgres=# select version();
version
--------------------------------------------------------------------------------------------------------------
PostgreSQL 9.4.4 on x86_64-unknown-linux-gnu, compiled by gcc (GCC)
4.4.7 20120313 (Red Hat 4.4.7-3), 64-bit
(1 row)

Thanks.