This keyword is used to get data values equals to IN clause values.
Example :-
Select * from test_table where field_1 IN ('123','abc','873','345')
Above SQL returns values equals to 123,abc,873,345 in field_1.
FIND_IN_SET
Suppose you have a String values like "123,abc,873,345". You can use FIND_IN_SET to get IN clause supported string as '123','abc','873','345'.
Select * from test_table where field_1 FIND_IN_SET (',','123,abc,873,345')
This SQL is also return above IN SQL results.
Check highlighted values.