How does the Rounding off Work

Hi there,
Could any one help me out ,how does the Rounding off mechanism work ,do we go from right to left or left to right and if so why?
The explanation in the book seems so vague

Here’s the question

Source: Manhattan 5LB book pg.400 Q.no.18

X is a digit in the decimal 12.15x9 ,if rounded to the nearest hundredth would equal to 12.16.

                                                 QTY A             QTY B
                                                    X                      4

https://www.youtube.com/watch?v=9AIlU52SInQ