What is Grandfathering clause?

Hi, I am assuming that you are referring to Grand-fathering clause related to taxation of Long Term Capital Gains on Equity Mutual Fund units and Shares. In general - A grandfather clause is a provision in which an old rule continues to apply to some existing situations while a new rule will apply to all future cases.  Grandfathering is a legal provision. Here it will mean that LTCG made up till 31 January will not be affected. Only the gains made after that date will be taxed. *Suppose you invested Rs 2 lakh in stocks or equity funds in March 2016.
*On 31 Jan 2018, the value of the investment was Rs 4 lakh.
*You sell the investment for Rs 5.2 lakh after 31 March 2018.
*LTCG will be calculated using the 31 Jan 2018 price. So the gains will be only Rs 1.2 lakh.
*Of this, Rs 1 lakh will be tax free in a year and only Rs 20,000 will be taxed at 10%. You may go through below articles :

• 10% LTCG Tax on sale of Stocks/Equity Mutual Funds | Budget 2018-19 Proposal
• Budget 2018 LTCG Tax on Equity Mutual Funds & Important Implications
• The grandfathering clause
• Equity LTCG Tax With Grandfathering Explained: Video + Calculator