If you're trying to import a multi-select field (with multiple values) or a bunch of tags, the field values should be separated by a semicolon (;) in the CSV/XLSX file.
For example: If you have an existing multi-select field called ‘Interested Products’ with multiple choices/values such as Freshdesk, Freshsales, Freshservice, etc., the ‘Interested Products’ column in the CSV/XLSX should be formatted as represented in the table below,