Properties is a menu in Studio used to change aspects of objects such as size, reflection, and transparency.
Opening the Properties Menu
To get to the Properties menu, open ROBLOX Studio, click on a project and press the "Edit" button. On the top of the window, click the "View" tab, and on the left hand side there is a button called "Properties". Click that, and the property menu will open on the right side the window.
How it Works
When you select an object in the project or in the Explorer menu, information on the object will pop-up in the Properties menu. The first column names the information (i.e. Size, Text, Name, etc.), and the second column allows the information to be edited. The Properties menu can change certain aspects of the object that is selected, for instance, changing the Transparency from 0 to 0.4 would make the selected object transparent. Properties are an important part to constructing various models and objects within a project, and should be familiarized with if you intend on making a game place, as developers tend to use it regularly.
How it is Useful
The Properties menu can be useful in many ways. One way is rather than having to use a script to insert properties into a part or basically anything, you can quickly and simply edit the properties through the menu. This keeps scripts uncluttered ad simplistic.