Expanded targeting lets Facebook automatically add similar interests to your Interest targeting to grow your audience.
Facebook only expands the audience if they think that this will help you get better results. It's not possible to see how much the audience size has exactly increased or how effective an expanded audience is compared to a non-expanded one.
You can enable this feature under Audience by selecting Use Expanded Targeting.
If you are using expanded targeting, the audience size estimate reflects the largest theoretically possible audience size after expansion. In particular, this means that when expanded targeting is enabled, adding or removing interests does not have any effect on the audience size estimate. If you want to get better visibility on how the interests affect the audience, you can disable the expanded targeting while you are designing your audience, and enable it after you are satisfied with the selected interests.
Expanded targeting is only available for Website Conversions and Mobile App Installs objectives.
Note: If you receive a permission error while using Expanded Targeting, it is because you have not been whitelisted by Facebook to use this feature.