Bounce rate depends on the measurement system
In Google Analytics 4, bounce rate is the percentage of sessions that were not engaged sessions. It is the inverse of engagement rate. An engaged session lasts longer than the configured engagement threshold, includes a key event, or includes at least two page or screen views.
This differs from the familiar single-page-session definition used in older analytics discussions. A person can read one page and still have an engaged GA4 session. Exit rate is a different metric and is not another name for bounce rate.
An illustrative calculation
If 100 sessions include 70 engaged sessions, the engagement rate is 70% and the bounce rate is 30%. This calculation describes the recorded sessions, not whether every visitor was an appropriate customer.
Interpret the result in context
A change can reflect the audience, page purpose or tracking setup. Check key events and the configured threshold before comparing periods. A contact page, technical reference and shopping journey do not necessarily need the same pattern of interactions.
Do not treat a low bounce rate as proof of more sales. Compare it with useful actions and the quality of enquiries or purchases.