Gizmos allow you to add graphics to the Scene View. By doing so, they help you to visualize design details that are otherwise invisible. There are various Gizmos that can be displayed within the Scene View, which can be selected or deselected via the Gizmos menu. You can find the Gizmos menu at the top-right of the Scene View toolbar, like here:

Once you have clicked on this, a drop-down menu will be displayed:

The Gizmos.DrawIcon function places an icon in the Scene to act as a marker for a special object or position. You must place the image file, which is used to draw this icon, directly in a folder called Gizmos. Otherwise, it cannot be located by the DrawIcon function.

Place the needed Asset files in this Gizmos folder or a subfolder within it. If you are placing an asset within a subfolder, remember to include the path in the Gizmos.DrawIcon function.

An example of Gizmos within Unity can be seen in the following screenshot (and their keyboard shortcuts):