WhatsApp Big Blue Dot - screenshot included

[CharlotteBog]

I have read all my messages.
What does the big blue dot mean? The last message is from them to me i.e. it's not a delivered/read notification (I know they are blue/grey ticks).

It's an iPhone

It's an unread message - did you mark it as unread afterwards or mistakenly swipe right on the chat in your lost?

*list

it means you've marked it as Unread

Oh, I did not know I could not that!
I knew swipe left (More, Archive), but not swipe right.

Thank you. It must have been a mistake.