b conventionally stands for the coefficient of the middle term of a quadratic expression.
The normal form of a generic quadratic equation in one variable x is:
ax^2 + bx + c = 0
Associated with such a quadratic equation is the discriminant Delta given by the formula:
Delta = b^2-4ac
The general solution of the quadratic equation may be written
x = (-b +- sqrt(b^2-4ac))/(2a)
or
x = (-b +- sqrt(Delta))/(2a)
Often people will assume that a is understood to be the coefficient of x^2, b the coefficient of x and c the constant term, and they will proceed directly from a quadratic equation such as 2x^2-3x+1 = 0 to speaking of something like b^2-4ac without telling you that a=2, b=-3 and c=1 are the coefficients.