Giving up on your first 2 cards for half your bet back.
Late surrender happens once the dealer is known not to have blackjack (with no hole card, a dealer blackjack afterwards takes the whole bet). Early surrender happens before the dealer's second card, and half comes back even against a blackjack. Some tables allow early surrender except against an ace. With 6 decks and S17, late surrender applies to 16 vs 9, 10, A and 15 vs 10.
At the table
At a table that offers surrender, a player with 10 bet says "surrender" after the first two cards, gets 5 back, and the hand is over. Some casinos also use a hand signal, drawing a line behind the bet.