What It Means
Profit booking means selling some or all of a position specifically to lock in a gain that's already there, not because the trader's view on the asset has turned negative.
Example
Say you bought 50 shares of a company at ₹800 each, a total of ₹40,000. Over the next two months the stock climbs to ₹960. Rather than wait and hope it keeps climbing, you sell 20 of those shares at ₹960, locking in a profit of ₹3,200 on that portion while letting the remaining 30 shares ride. That's profit booking: turning a paper gain into a real one, partially or fully, while the price is still in your favour.
Warning
Reading someone else's profit booking, or a stock dipping right after a rally, as proof the company's outlook has turned bad. Most of the time it just means traders who bought earlier are taking money off the table, not that new negative information has shown up.