You use a vector of partial derivatives, also known as [the gradient](https://en.wikipedia.org/wiki/Gradient). 

It is the direction along which the function has the largest increases.

With the descent algorithm, you take steps down the slope, 

 - each coordinate is updated according to it's derivative 
 - effectively that is like following the direction of the gradient.

Below is an example image from [this question](https://stats.stackexchange.com/a/487078/164061). The image shows how the gradient descent follows a path along the slope of the function, moving down to the minimum value. 

I have placed on top of it some extra arrows near the first step in the top. These arrows show the first step can be decomposed into two components, one for each coordinate. These steps are the single derivatives that you have in your equation.

[![example][1]][1]


  [1]: https://i.sstatic.net/oJrBx.png