See below for definitions and report terminology.

Active Users: 

The number of users that have been active within a 28-day time period.

The number of sessions that began on 

The length of time that was in the foreground; was in focus.

A new session is defined as a page view where no other session is currently active (the previous session has timed out).
A session ends (times out) after 30 minutes of inactivity. After a session timeout, a new session begins.
Note: Unlike previous versions, sessions are not restarted at midnight or when a new campaign parameter is encountered. 

Engaged sessions:
The number of sessions that lasted longer than 10 seconds, or had a conversion event, or had 2 or more page views.

Average Engagement time per session:
User engagement duration per session.

Engaged Sessions per user:
The number of engaged sessions per user (Engaged sessions divided by Users).

Engagement rate:
The percentage of engaged sessions (Engaged sessions divided by Sessions).
Note: Engagement rate replaces Bounce Rate in GA4.

The number of times a user triggered a conversion event.
Note: GA4 counts every instance of the conversion event, even if the same conversion event is recorded multiple times during the same session.