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.
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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