The Property Inspector is a panel on the right side of the application that allows you to set some common properties such as alignment, position and size as well as settings specific to certain controls (selection state, text properties, color, etc.). It is available when viewing mockups, assets, and Symbols.
You can show and hide the Property Inspector by clicking the toggle icon above it (keyboard shortcut CTRL/CMD + ; or ALT/Option + Enter). When hidden you can also show it by right-clicking a control and selecting "Properties...".
If you have multiple controls selected it will show properties that are common to all selected controls as well as options for aligning and distributing.
When no controls are selected the Property Inspector shows notes for the selected mockup, asset, or Symbol. For assets and Symbols it also shows the count and name of mockups where the selected object is used.