What exactly ad-hoc testing is? At what situation .

It's a random testing It means testing a application without proper test plan.It's carried out at the end of the project when all the test cases are executed.To check the application randomly. or it's carried out in CMM 1 level company where test are executed without planning

Testing the application randomly beyond the requirement to avoid the bug flow at the end users place is called ADHoc TESTING.

its a negative type of testing
we will never write any test cases for ADHOC.
if u found any bugs in adhoc try to convert into functionality/integration/system level bug
to re-test that bug we are going to write the Test Case because in future we may expect that bug.