A straddle is an optional bet that the player to the left of the big blind can make. The bet is live (meaning the player has the option to raise if everybody just calls the straddle) but it has to be made before the player sees their cards.
Think of it as a third blind and you're pretty much on track. It's usually twice the size of the big blind.
Note that straddles aren't allowed in tournaments and not all cash games allow them either. People usually make them to encourage action on the table. Some players will make them for the hell of it, others won't do it unless everybody else is doing it because they don't feel they get any advantage from it.