The TikTok Users' activity heatmap reveals when your followers are online on TikTok. The heatmap shows the busiest day of the week, as well as the busiest time of the day. The darker the tile on the heatmap, the busier the time/day is, in terms of when your followers are logged into the TikTok app.
Hovering over the heatmap you'll see the number of users for a given time of the day.
Note: The displayed time is based on the timezone selected for the profile. This can be found in: Profile settings > Reports > General > Timezone.