Can someone explain me the bold part of this Python code?

def f(x): return x**4 + 4 * x**3 + x**2 - 10 * x + 1

def gradf(x): return 4 * x**3 + 12 * x**2 + 2 * x - 10

def grad(f, x, delta=1e-5): return (f(x + delta) - f(x - delta)) / (2 * delta)

def g(x): return np.log(1 + np.exp(x))

def gradg(x): return np.exp(x) / (1 + np.exp(x))

def h(x): return g(f(x))

def gradh(x): return gradg(f(x)) * gradf(x)

x = np.arange(-5.0, 3.0, 0.01) y = h(x)

plt.clf() plt.plot(x, y)

def grad_desc(f, x0, alpha, gradf=None, eps=1e-5, maxiters=100): # initializaton, check arguments, ... x = x0 y = f(x) if gradf == None: gradf = lambda x: grad(f, x) xvals = [x] yvals = [y] for it in range(maxiters): d = gradf(x) new_x = x - d * alpha print("it=", it, "x=", x, "new_x=", new_x, "d=", d) if abs(d) < eps: break x = new_x xvals.append(x) yvals.append(f(x)) plt.plot(xvals, yvals, "o-") return x