Inpainting allows you to draw a "mask" on a given portion of an image and then use a prompt to re-draw the area within the mask and modify it. Here's how you can inpaint within Playground:
Hover over an image you've generated in Create and click on the "pencil" icon to Inpaint
Use your cursor to draw a "mask" (white shading) over the area within the image that you want to modify & click "Done" when complete. You can modify the stroke radius (brush size) of the mask with the slider, as well as erase parts of your mask and undo your last action as well.
Fill in the "Prompt" section by describing what you want to be generated within the mask and then click the "Generate" button.
The resulting image can be further modified if desired (Upscale, Face Restoration, Download, etc.).