What does the statement |b|=7
mean?

The absolute value of b is equal to 7. Absolute value means the distance from zero. In simple terms, the positive version of the number. B could be 7 or -7, since both will be equal to 7 when you take their absolute value.

It means b=±7, that is b=-7 and +7.  Because it is within absolute values brackets it can be either positive or negative as long as its magnitude is equal to 7...