Unclear documentation - Metadata Refresh

We use NAS source to store CSV files. Daily an external jobs feed a folder to add new CSV file. We applied a schema at the folder level because all CSV files have the same format.
When adding a new file, new data is not available and we don’t understand how settings on NAS source apply in conjonction with ‘Alter Table Metadata Refresh’ command !
Documentation on NAS source says :

What means Expire after ? based on what time ? when is the starting time (after alter table command) ?
Does Alter Table have anly link with those parameters or is it totally independent ?

@Arnie Metadata refresh is background and happens for all datasets in the source

ALTER PDS REFRESH METADATA happens just for the table on demand