Bounce Rate is defined as the percentage of visitors that leave a webpage without taking an action, such as clicking on a link, filling out a form, or making a purchase.
It means that, take this example for better understanding. If you have a shop in the market and customers come inside and exit immediately than what do you think about them.
You think that they are not interested in your products or services that's the same case with google when he tries to rank website according to users query then It considers bounce rate as a major factor. So you need to make it your top priority for better results