The scratchpad is a temporary working space for keeping commonly used shapes for easy access. It can be found at the top left of editor, above the libraries. If it is not shown, you can display it using the menu under view -> scratchpad. You can also hide it by clicking the "x" icon by the scratchpad.
To store elements in the scratchpad, drag and drop them from the diagram to the content area or click on the "+" icon while the elements are selected. To insert elements into the diagram, drag and drop them from the scratchpad to the diagram as follows:
To reorder or remove entries, click the pencil and use the dialog that appears:
Note that the scratchpad is stored in the local storage of the browser, which means it's contents are deleted if cookies are cleared. The scratchpad is automatically saved between each change.
You can export the scratchpad as a draw.io library from that dialog, if you want to store it more permanently or share it with others.