Messages List - What does the red line to the left of messages mean?

We have a multi-day gap in the logs, although we believe that the application was working at that time, and a red line is visible in this gap.
What does it mean?
Thanks

The left colums shows a 'time distance indicator'. This is intended to help you intuitively recognize the time between two messages without having to first look at the times closely.
The time gap is indicated by different line length, thickness and color.

So, the message times say that there was a big time gap between the two messages. And exactly this is also shown by the red marker.