The property bar is the panel just below the menu bar. It really consists of two tool bars, one on the left and one on the right. The contents of the tool defaults section (left bar) depends on the selected tool, and the contents of the object property section (right bar) depends on the selected objects. An enumeration of specifc tools and their editable defaults and specific objects and their editable properties is contained in the Tools and Objects section of the documentation.
If multiple objects of a different sort are selected, the properties that become editable in the bar are the common properties of the types of objects selected. For example, suppose an edge and a node are selected. An edge has editable properties in its line color and number of arrows. A node has editable properties in its line (outline) color, fill color, and a lot of other stuff (none of which is the number of arrows). If both an edge and a node are selected, only the common properties, in this case the line color, will have the button to have its property edited displayed.
So, without further ado, editable property icons. When the short description doesn't cover everything, more explanation is given.
For objects that have multiple text fields like nodes, these are buttons to add and delete those fields.