SNAP 9 Help is not working for some of us, but you can also refer to NASA SeaDAS 8.2.0 Help. Try a search using site:https://seadas.gsfc.nasa.gov/help-8.2.0/ "CSV" "Import".
Note that some tools (e.g., Sen2Coral) require a different “CSV format”.
Please have a look at previous reports of problems importing data in CSV format. If your problem is not covered in previous posts, please provide enough detail to allow others to reproduce your problem (see the FAQ along with the first few lines of the CSV file.