Straddle is the optional third blind, usually posted by the player in the Under the Gun position for double the amount of the big blind. Sometimes, a straddle can also be posted by a different position, often a button, but some games will allow players to straddle for any position, with their straddle being live if there is no action ahead.
A poker straddle has a goal to increase the stakes and generate action. With more money in the pot to start, there is more reason for players to get involved.
As for the straddling player, for the money they put in the pot voluntarily, they usually get the benefit of acting last before the flop. This alone isn’t enough of a compensation, which is why straddling is usually not a good strategy unless the entire table agrees to do it. In that case, it simply becomes a larger game with an extra blind.
For example, in a $2/$5 game, the small blind will post $2, and the BB will most $5 before the cards are even dealt. The UTG player decides that he wants to straddle and put $10 (double the BB) into the pot, also before cards are dealt. In this case, the preflop action starts with the UTG+1 player, and the player in the straddle will be the last one to act in this round. Postflop rounds go as standard, and straddling player will not have any advantage there.