The share of visits where somebody arrived, did nothing measurable, and left. In GA4 the equivalent is the inverse of engagement rate — a visit counts as engaged if it lasts ten seconds, fires a conversion, or sees a second page.
Bounce rate has a reputation as a quality score for a website, which it is not. It is a behaviour measure, and whether a high figure is bad depends entirely on what the page was for.
A contact page where people read the phone number and call you has a terrible bounce rate and is doing its job perfectly. A service page with a 90% bounce rate is a different matter — nobody is going any further, which usually means the page did not match what they searched for.
This comes up most often in Analytics & Conversion Tracking. Analytics, conversion events, call tracking and reporting installed and verified, so every enquiry can be traced back to what produced it.
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