What is the role of #fsync and #synchronous_commit in configuration file .

Hi ,
I want to imporve  the performance for inserting of huge data in my table .
I have only one idex in table .

First question - i want to know the role played by

 #fsync   = on    and
 #synchronous_commit = on

They are commented by default in 8.4 .
When made like this :-
fsync = off             
synchronous_commit = off     


It improve the performance :)
and query took less time .

I want to understand more in details what exactly had happened  one is made them "off" , is it dangerous to do this ?  as it will not sync the data in each commit .

Pls help me out .

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Thanks,
Keshav Upadhyaya