You win a big hand, you bought in for $100 but you've got $500 on the table, woohoo! But you're scared of losing it, so you take $200 and slip it into your pocket, locking up a win but preventing anyone else from winning their money back. Illegal in most cardrooms, and frowned upon pretty much everywhere. Other players don't like to see their money leave the table without a chance at getting it back.
Soft way of doing it is just putting it in a pile to the side of your stack and not touching it, only putting your profit into play. Some games don't like this. Once had a player teaching my apes to do that, ridiculous, had 3 chimps locking up a "win" of $2 in my 5c/10cent game.