As we can see that the boy is moving his hand up and down while producing the wave in the string.
Here we can see that the source of disturbance or source of wave here in this example is the boy who is holding the string at one end.
So here the motion of the hand of boy is representing the motion of medium molecules in the medium always
So here the disturbance of wave is travelling in the string to the right direction while if we see the direction of medium molecules then it is moving perpendicular to the string i.e. up and down
This type of wave is known as transverse waves in which medium molecules moves perpendicular to the direction of wave
