Funnel's custom metrics use basic arithmetic to let you create your own formulas based on any of Funnel's standard metrics or any of your data sources' native metrics that are being downloaded.
Google's custom metrics
If you're familiar with Google Data Studio, you'll know that custom metrics can be created as custom fields when viewing the Data Source fields, buy clicking on the blue plus button at the top of the table:
The custom metric builder in Data Studio isn't the most user friendly as the formulas can be quite complex. If you're currently not using Funnel, you'll also only be able to create custom metrics for one given ad platform/data source.
Funnel's custom metrics
Funnel has a built in custom metric builder which simplifies the process. Once the metric has been defined, the new metrics can be viewed in Data Studio after refreshing the data source fields.
As apposed to creating metrics for an individual ad platform, you're also able to combine metrics from multiple sources.