The distance moved by a particle executing simple harmonic motion in one time period is
One Time period is the total time in which particle returns to the same point and in the same direction from where it is pushed.
Now, In Simple Harmonic Motion, Total distance traveled is 4A.
This is because, if the Particle starts from the mean position then, it will move to one extreme, covering a distance A(Amplitude), and then returns to mean position, and hence covering another distance of A.
But direction is opposite, and one time period means that direction should be same, therefore, it will move in opposite extremes, and hence covers a distance A and then returns to the means covering a distance A. In this case, direction is same as it was first pushed.
Therefore, Total distance=A+A+A+A=4A