It's used to describe something that doesn't belong or fit in, that's in-between, undefined, or ambiguous. It can also be used to describe someone who is not committed to either side of an issue or situation.
Here are some examples:
* "El estilo de la película es ni de aquí ni de allá." (The movie's style is neither here nor there.) - This means the style is not easily categorized or defined.
* "Él no se pone de parte de nadie, está ni de aquí ni de allá." (He doesn't take sides, he's neither here nor there.) - This means the person is neutral and doesn't support either option.
* "La situación es ni de aquí ni de allá, no sabemos qué pasará." (The situation is neither here nor there, we don't know what will happen.) - This means the situation is uncertain and unclear.
In short, "ni de aquí ni de allá" is a phrase that highlights the lack of clarity, definition, or commitment to any particular side or position.