You can check this in some Easy way :ORACLE-BASE - Oracle Cost-Based Optimizer (CBO) And Statistics (DBMS_STATS)
>> It Any role in oracle performance tuning? Does it make good performance of Oracle DB??? :Yes
I have a couple of spastics as well in my team, they can type really fast, but the database isn't performing....
You'll need to carefully read and understand the documentation.
This isn't something one can explain to you in 2 sentences, it is pretty complicated stuff.
Actually, I think the most recent release (12.1c) has it documented better, especially the Oracle® Database SQL Tuning Guide
12c Release 1 (12.1) which you can find here: Contents