An actor transform preset is the full actor transform (location, rotation, and scale) that you see in the transform bar on the bottom of the viewport. These presets can be assigned to the selected actor or unassigned and used as an arbitrary positioning preset. These can then later be recalled very quickly from the preset panel on the right side
Select the icon in the side bar
Alter preset name as needed
Actor association means the transform value & actor will be saved together. Unassociated presets mean only the transform value is saved and can be applied to any actor.
Click Save and your Preset will now appear on the right side preset panel
Presets can be deleted by selecting "Edit / Order Presets", selecting Edit on the preset, and then selecting delete.