The second derivative of position wrt time is the same as acceleration, so all this equation is saying is ma = -kx, which means that F = -kx, which is Hooke's Law. To understand what this means you first need to know what x is defined to be. It usually means displacement from the equilibrium position of the spring. Also, "F" usually means the force exerted by the spring. If we ignore the "-k" for a second, we see that the Force is proportional to displacement, which means the more the displacement of the mass, the larger the Force exerted by the spring. The "k" is the constant of proportionality and the minus sign indicates that the direction of the force is opposite to that of the displacement (i.e if you pull the mass to the right, the spring tends to pull it back to the left, which makes sense intuitively).