clustering factor

I use Oracle 8.0.6 with SAP R/3. I have a lot of indexes with big values for "clustering factor" (998745 for an index of about 12 million rows). I remember i've read something on this item and it has something to do with performance.
I don't remember what this means and i've never found further explaination on it since this moment.
Could someone explain

1. what it has to do with performance problem (if it has) ?
2. what is bad ? a big number or a little one ?
3. How can i solve it ?

Regards,

--
Thierry HUGE
AIX/Oracle/SAP Administrator
STEELCASE STRAFOR (FRANCE)


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