Are you saying that you do not know what "modulus" or "absolute value" means? |x| is equal to x if , -x is x< 0. In particular that means that |x- 80| is equal to x- 80 if [itex]x\ge 80[/b] and equal to -(x- 80)= 80- x if x< 80. Saying that |x- 80|< b means thateitherx- 80< bor-(x- 80)< b. That last is the same as x- 80> -b. Putting them together, if |x- 80|< b then -b< x- 80< b. That is, |x- 80|< b as long as x- 80 lies between -b and b. If you want you can add 80 to each part to say that if |x- 80|< b then x itself lies between 80- b and 80+ b.

Essentially, that says that x lies in an interval having 80 at its center and extending a distance "b" on either side. We can think of |x- 80| as a "distance". |x- 80|< b means that the distance form x to 80 is less than b.