Macros are a collection of a particular item, such as email addresses (Email Macros) or login names (User Macros).
Macros can be used in areas of the product that apply.
For instance, you can use a single Email Macro containing 10 email addresses in any Email Alert.
So rather than defining 10 email addresses in the Email Alert definition, you define it once in an Email Macro under a more convenient alias. The added benefit is it can be re-used for other Email Alert definitions if needed.