Field aliases enable you to rename data source fields. For example, you could rename 'Task Count' to 'Número de tareas', rename 'Invoice/CR Line Total' to 'Sales $', or anything else that may make sense to you and help make the meaning of fields more clear and relevant to your users as they build reports.
By default, field aliases are pulled from the prebuilt templates in Easy Insight if any aliases exist. Field aliases only apply to the display of fields when choosing fields in the report editor--calculations will still use the original field names to ensure that nothing breaks in your scripts.
You can customize these field aliases by going to data source configuration -> Field Aliases. The toggle at the top allows you to choose whether or not you're customizing field aliases:
First, choose a child data source, then enter aliases for each field you want to alias:
After saving, you'll see your updated field names when you go to choose fields in the report editor: