Definition of burn
The expression is often used to talk about "burn a card", i.e. to remove it, to discard it. The technique of shuffling cards will vary depending on where you play, but often it works like this: The dealer shuffles all 52 cards. When he has finished his shuffle, he presents the deck to the player to his right. He presents the deck to the player to his right because he is the last player to receive his cards. This player cuts the deck, that is, he will split it into two equal or unequal parts. The second part of the packet will be put back on top of the packet to change its order. When this is done, the first card on top of the deck will be burned, i.e. it will be discarded into the discard pile. The player to the left of the dealer will receive the second card.
Burn one card, extra security
All of these techniques are safeguards to ensure that no one cheats or that no one can make a rigged shuffle of the package to benefit themselves or a friend. The fact that burn a card makes cheating more difficult. Even in some casinos that have automatic shufflers, the first card is often burned. Some dealer's also go burn a card before dealing the turn and the river.