Quick Inserts are nodes that can be inserted after a selected node with a single hotkey. These are among the most commonly used nodes. These nodes will change depending on which Node Editor type you are in. Each hotkey is displayed in the icon of each Quick Insert button for reference. It makes more sense to use the hotkeys here but they’re also available in the menu if needed.
The newly inserted Color Ramp node. The Quick Inserts feature does its best to determine which output socket should connect to the adjoining node socket. It searches for like types first. In this example the Noise Texture node’s Fac socket was connected to the Base Color socket of the Principled BSDF node before the button was pressed, so the Color socket of the Color Ramp node was used to link to the Principled BSDF.