A standing wave is the sum of two progressive waves which are propagated in opposite directions. In order to obtain 2 progressive waves of precisely the same frequency, and being propagated in opposite directions, one must use a device that is capable of reflecting an incident wave without deforming it. Sort of like a mirror for light.
This is what we observe here. An incident, progressive, wave meets with a modification of its medium of propagation. Conditions at the limits must be taken into consideration. In the particular cases of free or fixed extremities , we obtain a total reflection, as shown in the animation.
The resultant wave, which is the sum of the incident and reflected waves, is a standing (or stationary) wave. Certain points (nodes) remain stationary at all times while others vibrate with maximum amplitude (anti-nodes).