I played FE Sacred Stones, Blazing Sword and Blinding Blade (until ch13) before playing FE Heroes. I can't wait for Tana to be out on FEH.
Anyways, I've been checking guides on Gamepress on tiers and stuff and they mention that some heroes have "Low BST".
I may not have looked hard enough but I couldn't find any place explaining what it is. I haven't read about it in League of Legends or other games I've played either.
My best guess is "Burst" but I don't think there's much point in removing just 2 letters.
Best Answer
From what I've read in several pages I guess the BST is some kind of internal value the game assigns to a character to quantify it's potential. Allowing to establish some kind of balance for the Arena and other multiplayer game modes.
This is the link that helped me the most on understanding this. Since I'm not able to find an oficial description of the concept I would suggest to wait for a more accurate answer.
Edit:
Just found out this page where explains how the Arena and some of it's mechanics actually works and mentions the BST:
Also in this page is included a link to a list where you can check the BST of each character. The web page appears to be in japanese but it includes the characters portraits.