Make sure you’ve read the general notes about all the nodes: Reference - nodes

Map to Blender image

This node draws a map into a Blender image.

Choose a name for the Blender image to use, choose the resolution, and click the draw button.

There are several map inputs. None of them are mandatory. The RGB input allows you to have a grayscale image without having to connect all the R, G, and B input channels individually, and is also faster. The individual input channels have priority over the RGB one. So you can draw different maps into different channels of the same image.