Expected utility hypothesis is kind of like an equation that helps us figure out what decisions we should make. It can help us decide between two options and figure out which one is the best.
Basically it works like this--when you're choosing between two options, it looks at how 'beneficial' each option is to you. It then assigns a number to each option, which is called its "utility". The higher the number, the more beneficial it will be to you.
Finally, the equation will 'expect' that you will choose the option with the higher utility. So it will help you make the best decision in a situation!