Line meaning

Many thanks Hengel.
Is it returning one array per relevant spreadsheet columns, by such line?

What is the purpose to 'care' about only 'First()' below?

                                                    IEnumerable<DocumentFormat.OpenXml.Spreadsheet.Column> ic = cs.Elements<DocumentFormat.OpenXml.Spreadsheet.Column>().Where(r => r.Min == columnIndex).Where(r => r.Max == columnIndex);
                                                    if (ic.Count() > 0)
                                                    {
                                                        DocumentFormat.OpenXml.Spreadsheet.Column c = ic.First();
                                                        double cc = c.Width;
                                                        sw.WriteLine("Cell width: {0}", cc.ToString());
                                                    }

Select allOpen in new window

First() returns what the name is suggesting: the first (and eventually the only) item from the collection.
this avoids to "loop" on the collection.

To the original line, why does it need to check both Min and Max in there?

I don't know. that is the application logic presumably
it surely tries to consider only cells that are not merged, and at exaclty that position

Is such line only destined to check for non-merged cell? How to detect that relevant cell is a merged cell?

Thanks a lot Hengel. Where is the documentation located at?

sorry, I had not pasted the URL above:
https://msdn.microsoft.com/en-us/library/documentformat.openxml.spreadsheet.column(v=office.14).aspx

you can find such information really quickly, using google, you search this:
DocumentFormat.OpenXml.Spreadsheet.Column site:microsoft.com