A CSV file holds a table as plain text, with each row on its own line and the columns separated by commas. Because it has no formatting or formulas, almost every spreadsheet, accounting package and database can read and write it.
That makes it the everyday route for moving lists between systems: exporting customers from one tool, or importing a supplier price list into another. It has limits, though. Dates, leading zeros and special characters can be mangled on the way, so a sensible import checks a sample before loading everything.