What Does the Number 1 Next to Nabla Symbol Mean?

[bezitor1]

I have the number 1 next to Nabula and I do not know how to solve it. For reference, I am a Korean person, so it would be very difficult if you explain it difficultly.

 

[PeroK]

You have a function of two vector variables $\mathbf{r_1} = (x_1, y_1, z_1)$ and $\mathbf{r_2} = (x_2, y_2, z_2)$.

You can take the gradient with respect to either of these. So $\nabla_1$ is the gradient with respect to the first vector variable and $\nabla_2$ is the gradient with respect to the second vector variable.