Please explain what is BST streak finisher badge meaning?

Answer 1

BTS Streak Finisher I think you meant.
There was a contest/challenge "Back to school" created by sololearn about a year ago.
The Back To School Streak Challenge is a virtual challenge that takes place between August 20 and October 1, 2020.
The rule was to complete a lesson a day for 15 straight days. You can't miss it or you have to start over. If you succeed, you would receive a certificate, badge, and one month free of SoloLearn PRO. Plus was a chance to win an Apple Watch and more!
Here is the detailed information:
https://get.sololearn.com/back-to-school/
https://get.sololearn.com/back-to-school/contest-rules
It's finished now ;)

Answer 2

Edit:
From recent news, sololearn declares that the back-to-school contest or a similar event is coming soon.So you have to wait for a little more :)