Definition of straddle
The straddle is like a third big blind. In some casinos, in games of cash games, you will have the option (if you are to the left of the big blind) to pay a bet twice the size of the big blind to have the privilege of speaking last preflop. For example, if you arrive at a table of cash games with a blinds at €1 to €2, you will have the option to pay a straddle equivalent to 2 big blinds (€4) to be in the hand.
Paying the straddle, a good idea or not?
Unless you're in a really special situation, the straddle is not profitable in the long run, even if you're a very good player. There may be exceptions. For example, if a player in the middle of a tilt is playing a puck on the table, you may want to get into the hand as soon as possible to have the opportunity to take all of his chips. But under normal circumstances, it has been mathematically demonstrated on huge sample hands, even with very good players, that paying straddle is a losing game in the long run.