Expression targets allow you to create sophisticated targets once and use them repeatedly in different campaigns, experiences, and so on. In an expression target, you can also use multiple OR and AND conditions that are not allowed in the standard targeting interface.
For best practices, see Script Profile Attributes.
To create your own reusable expressions in targeting groups:
Example: Suppose we wanted to target customers on Saturdays and Sundays in order to offer them promotions only available on weekends.
return (new Date()).getDay() == 0 || (new Date()).getDay() == 6;
Saturday OR Sunday (In server time, 0 is Sunday and 6 is Saturday.)
You can now use your target in campaign and experience population selection, in segments, or in success metrics.