Engaged Sessions

One of the three is enough. Someone who opens two pages in three seconds is engaged. So is someone who reads a single page for a minute and leaves.

A key event is any event you've marked as important in GA4, like purchase or a form submission. Key events used to be called conversions.

The 10 seconds count only while your page is in front. That's engagement time, not time on the clock, so a page left in a background tab adds nothing. User Engagement shows how that time is measured.

The 10 seconds are only the default. You can raise them in GA4's settings, up to 60 seconds, and the change doesn't affect data you've already collected.

From here come two numbers. Engagement rate is the share of sessions that were engaged. Bounce rate is the share that met none of the three. Bounce rate isn't in the default reports, so you may need to add it yourself.

So a low Engagement rate isn't always bad news. A quick answer read in eight seconds still counts as a bounce, so judge it against what the page is for.