Grandfather Clause is basically a provision in which an old provision continues to apply to some existing situations while a new rule will apply to all future events. Those who are exempted from the new rule are said to have acquired grandfather rights.
There has been a grandfather clause in the previous financial year stating that all the capital gains prior to 31st January shall be grandfathered, which means that the new rule shall not be applied to the exempted categories. The tax bracket has been 10% and no indexation benefit was allowed.
So, generally speaking, that the grandfather clause (if any) helps to keep tax gains at par with old rules. Recently no grandfather clause has been added, it was applicable only to the previous financial year.