An option is a bet on the future price of a stock. If you think that Stock XYZ will go from $15 today to $20 next month, you can buy an option that will reflect your prediction.
A multi-leg option allows you to bet on the "magnitude" of a stock's price change, rather than the "direction".
For example: "Tesla reports earnings next month. They're either gonna be REALLY good, or REALLY bad. So let me buy a multi-leg option that predicts a 10% price jump either up or down". You'll make money whether Tesla goes up 10% or down 10%. You'll lose a ton of money if Tesla remains roughly the same price.