When you straddle something, you're sitting on it with one leg on each side — like straddling a horse or a fence.
Unless you're using an old-fashioned side-saddle, you straddle a horse when you ride it. Gymnasts learn how to straddle the parallel bars, basically doing the splits on them. But if someone says you're "straddling the fence," it means you're doing a different kind of split: you're not taking a side and refusing to commit. In finance, straddling means you want to leave your options open to buy or sell.