What are the key metrics in Google Analytics and what do they actually mean?

Acquisition:

• Users — unique visitors. GA4 defaults to active users, meaning those with an engaged session, which is not the same as total users.
• Sessions — a group of interactions. In GA4 a session ends after 30 minutes of inactivity but, unlike Universal Analytics, does not restart at midnight or on a campaign change.
• New versus returning users.

Engagement:

• Engaged sessions — over ten seconds, or with a conversion, or with two or more pageviews.
• Engagement rate — engaged sessions divided by total sessions. The replacement for bounce rate, and a better measure.
• Average engagement time — time the page was actually in focus, which is more meaningful than Universal Analytics' time on page, since that could not measure the final page of a visit at all.
• Views per session and event count.

Outcome:

• Conversions / key events, conversion rate, revenue, and average order value.

Note: The interview point is which metrics matter. Traffic volume is rarely the answer — a site can double its users and earn less. Conversion rate by channel, and revenue per user, tell you whether growth is worth having. Being able to say that engagement rate is a diagnostic and revenue is the goal shows the right instincts.