A straddle is a voluntary bet in the amount of two big blinds that a player in the position after the big blind (and only in that position) can make preflop in a game at the cash tables.
If there is a straddle, the next player can also make a blind bet, but in the amount of 4 big blinds (2 times the straddle). Such a bet is called a re-straddle. The next player can increase the bet by 2 times more by placing another re-straddle. The last re-straddle can be made by a poker player on the button.
The player who made a straddle or a re-straddle gets the right of last word preflop. Basically, a straddle can be compared to buying the big blind's position. Decisions on voluntary blind bets are made before the cards are dealt, which takes a few seconds.
If a new player joins the table, they can also make a straddle or a re-straddle if their position allows it.
A player can always skip the straddle and re-straddle by selecting the appropriate option in the table settings.