"Gradient descent is a first-order optimization algorithm. To find a local minimum of a function using gradient descent, one takes steps proportional to the negative of the gradient (or of the approximate gradient) of the function at the current point."
Gradient descent is based on the observation that if the multi-variable function $F(x)$ is defined and differentiable in a neighborhood of a point $a$ , then $F(x)$ decreases fastest if one goes from $a$ in the direction of the negative gradient of $F$ at $a$, $-\nabla F(a )$. It follows that, if
$$a_{n+1}=a_n-\gamma \nabla F(a_n)$$
for positive $\gamma$ that is small enough, then $F(a_n) \ge F(a_{n+1})$.
