Ok, I'll give you explanation:
When a project work with list or table which bound to file, program loads file data into RAM. After finishing working with such list/table, program unloads its data to file on hard drive.
And the timeout means that within this time file will be unloaded from RAM to hard drive.
Thank you VladZen for your explanation. Now I understand what this setting means. Maybe I should set this to a lower timespan because some of my projects with a bunch of tables and lists eat up much memory...