An old pond
A frog jumps in
The sound of water
This is not supposed to be a particular pond, and so it is empty of adjectives or a name.
The poem is not about a special place, but the specialness of a moment.
The suddenness accentuates the stillness and focuses our attention on the moment.
The action comes and goes, we cannot hold on to it, just as in real life we never see these frogs jumping in ponds, we only know that it happens.
The sound of water is also the sound of a door closing.
That moment ends and we are left in the present moment.
This poem points at the present moment. The moment we are always in, now. We forget sometimes and get caught up in the past and the future. We are always here and now.