Understanding an IC, MP1584, error amplifier
It is a loop compensation node, it adjusts the feedback loop gain to keep the system fast and stable.

Determining appropriate component values requires quite some elaboration. You need to use an averaged model of current mode buck controller to make bode plots of control-to-output and output-to-feedback. Then you insert poles and zeros to get the required phase margin. Since it is a current-mode controller, one pole should be enough.

If you want to dig deeper into this subject, take a look at Christophe Basso's books.