It's often used to describe something that is currently fashionable, popular, or in vogue. For example:
* The restaurant of the day: This could refer to a restaurant that is currently very popular or trendy.
* The color of the day: This might refer to a particular color that is fashionable at the moment.
* The dish of the day: This refers to a special dish that is featured on the menu that day.
Essentially, "de jour" implies something that is "in style" or "the current favorite."