Active users in last 30 minutes shows activity as it happens using data from the Realtime report. The first half of the card shows the total number of active users in the last 30 minutes and the number of active users in the last 30 minutes per minute.
The second half of the card includes realtime activity for a relevant dimension. For example, the Engagement overview report may show the top pages and screens users are visiting right now, while the User attributes overview report may show the top countries users are visiting from right now.