Once you have some idea of how long a typical query statement should take to execute, this setting allows you to log only the ones that exceed some threshold you set. The value is in milliseconds, so you might set:
log_min_duration_statement=1000
And then, you'll only see statements that take longer than one second to run. This can be extremely handy for finding out the source of outlier statements that take much longer than most to execute.
If you are running 8.4 or later, you might instead prefer to use the auto_explain module at http://www.postgresql.org/docs/8.4/static/auto-explain.html instead of this feature. This will allow you to actually see why the queries that are running slowly are doing so, by viewing ...
