What's a data table?
On this website, a data table is a plain text file containing records broken into fields separated by tabs, with 1 record per line. In other words, it's a TSV, or 'tab-separated value' file. See this page for the reason the recipes are based on TSVs, and this page for instructions for converting a CSV to a TSV.
The data in the records are strings of letters, numerals, punctuation and spaces. Each field in a record contains just 1 data item. The first record is the header and contains the names of the fields.
An example is shown below as it looks in the Geany text editor. This data table has 5 fields and 11 records. The 5 data items in the header are field names. Some of the data items in the 'Mobile' field are empty strings — they're missing, but the table holds places for them.
On Linux and Mac computers, a data table file doesn't need a filename suffix, and can just be named table or something equally simple. On a Windows computer, the filename could be table.txt or table.tsv or table.tab. I occasionally see a Windows file named [something].csv which is in fact a TSV. The important thing is that the file is plain text and structured as described in the definition of a 'data table', above.