What actually the meaning of "-n" in sed?

Question

According to http://linux.about.com/od/commands/l/blcmdl1_sed.htm

suppress automatic printing of pattern space

I've tested with or without -n, sed will produce same result

I dont understand what space does it means.

Answer 1

Just try a sed do-nothing:

sed '' file

and

sed -n '' file

First will print whole file but second will NOT print anything.