CSV stores tabular data in plain text. The first row is usually a header; every following row has the same number of fields, separated by commas (or tabs, or semicolons, there's no real standard).
Fields containing commas, quotes, or newlines must be wrapped in double quotes, and internal quotes are doubled (`"`).
Every spreadsheet app and database can import/export CSV, which is why it survives despite being a mess: RFC 4180 only standardized it in 2005, decades after it was invented.