What does it mean?

The constitution gives the President the power to return a bill unsigned but it circumscribes the power to send it back only once for reconsideration.
If the Parliament sends back the bill with or without changes, the President is duty bound to sign it. However, deliberately or inadvertently, the constitution does not set a time-limit in which the President is obliged to approve the bill, so they may withhold assent indefinitely. This has come to be known in legal and constitutional circles as the “Pocket Veto”, and has been used on a number of occasions against controversial Bills.

Use in India

• In the mid-1980s, President Zail Singh withheld assent to a Bill passed by Parliament that gave sweeping powers to the State to intercept mail. This was considered by the President to be an encroachment on citizens’ freedom of speech and liberty as guaranteed by the Constitution.
• In early 1990, President Venkataraman withheld assent to a Bill passed by the outgoing Parliament that gave pension benefits to themselves. This was interpreted by the President to be self-aggrandisement.

Source: Sambhav Garg