If you still don't get some part of what's going on after copying it and playing with it a bit (A1 is the code, A2 is the input, you can find an explanation of

If you still don't get some part of what's going on after copying it and playing with it a bit (A1 is the code, A2 is the input, you can find an explanation of BF and a simple Python implementation in my linked slides), try reading my discussion with briansteffens, who is also the guy who wrote the Apps Script implementation that inspired this.