Program member custom fields are fields that can hold program data for each member that are accessible inside a specific program.
The PMCF fields can be used in different Marketo assets as mentioned in below links:
How to import values to PMCF fields using List Import:
The List import activity is similar as any other list import, the only difference is that the PMCF values should be imported from Program Members tab instead of a Static List.
The PMCF values are now imported to the Leads in the Program with status as Member.