Hi Marco, that means the thread is marked as "High Interest" for you, it's a per-user flag. You can modify it by using "Edit thread options" at the bottom of the content area, or a shortcut is to click to the left of the thread title in the threads list. You should see that the ones with the icons are marked in "User Options" as Interest: Interested and the ones without are Interest: Normal.

re:
> but I can't undertstand why it is displayed also for a thread like "Strata sculpt..." that has only 14 messages.

You've probably posted messages in those particular ones? There is a setting relating to that under
Options > Forum Options > Display > Automatically mark threads I post in as High Interest

Since it's a per-user flag others will see different ones marked than you.

- Michael