Help me to understand (documentation)

Hello.
I translating the PostgreSQL manual and I run into the problem. I don't
understand this:

---------------------From
queries.sgml--------------------------------------------------------------------------------
The <literal>ON</> or <literal>USING</> clause of an outer join is
<emphasis>not</> equivalent
to a <literal>WHERE</> condition, because it results in the addition of
rows (for unmatched input
rows) as well as the removal of rows in the final result.
--------------------------------------------------------------------------------------------------------------------------------

I don't understand the rest of the phrase after "because".
Help me please. Please write same, but in other words, may be I'll
understand in this case.

With best wishes,
Victor Vislobokov
St.Peterburg, Russia