Bounce rate is a metric of user interaction on a site. In analytics-speak, bounce rate refers to the percentage of single-page visits or interactions. In other words, it's the number of visitors who come to your site and leave without taking any further action.
For example, someone might land on your homepage from a Google search. However, if they quickly leave without doing anything else – clicking on a link, going to another webpage, filling out a form, etc. – that would count as a bounce.
But what determines a bounce rate is very different for both GA4 and UA.