Properties are information about a particular screen element that tells it how to look or how to behave. Every screen element has what are called size and position properties, which has information on how large or small the element is (its size) and where it is on the screen (its position).
Then there are properties which are unique to certain screen elements. For example, text has font and italic properties which, as you might expect, dictate the font the text uses and whether it’s written in italics or not. Other examples include: buttons have an action property which says what should happen when the button is pressed, and an input element (that the player can type into) has a length property to dictate the maximum length the inputted text can be.
n general, when adding properties to an element or as part of a style, you will simply write the name of the property (e.g. font), then a space, and then the value of the property (e.g. "ComicSans.ttf"). Properties always come in <property name> <value> pairs (e.g. font "ComicSans.ttf"), and you can’t split up a property’s value onto a separate line from the property name.
See also Ren’Py Screen Language Basics – Images and Text for a detailed explanation on organizing properties for screen elements.
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