Delectus can import data in CSV format. CSV (or "comma-separated values") is a common format for exchanging data among database and spreadsheet applications. Most database and spreadsheet programs can export data as CSV.
There's no special "import" command in Delectus. If you want to import CSV data, simply find the CSV file and open it in Delectus. Delectus automatically converts the CSV data to its own format.
You should make sure that the CSV file has the labels of its columns in the first line, and that no two columns have the same label.
Programs that export their data as CSV usually provide a way to say whether to put the column labels in the first line; just make sure you use that option when exporting your data to CSV.
Most programs that can export CSV have the same requirement that each column must have its own label, so in most cases you won't have to do anything special to ensure that your data can be read by Delectus.