This property on the data sources listings has to do with how Funnel collects and stores data for a data source.
For most data sources, data is collected and stored in chunks based on specific dates. In these cases Funnel displays the date range for which Funnel has been able to successfully collect data.
In certain cases (like in the case of self-managed Google Sheets sources) data is collected and stored in one single file. For these types of data sources a date range won't be displayed and instead Funnel states whether (all) data has been successfully imported.