How to display files in a grid with details?

The problem here is that you will need to again query all the documents in other column and then use document() function on it. 
You don't have those documents as part of your data subset. So either you add it in your data or you query the folder again to get a list of documents. 

This is my code with the Description column and I recieve an error message:

load(
            local!data: {
              {
                id: 1,
                folderId: 4768/* Folder ids*/

              },
              {
                id: 2,
                folderId: 4947/* Folder ids*/

              }
            },
            a!gridLayout(
              label: "Documents",
              labelPosition: "ABOVE",
              headerCells: {
                a!gridLayoutHeaderCell(
                  label: "Name"
                ),
                a!gridLayoutHeaderCell(
                  label: "Description"
                )
              },
              columnConfigs: {},
              rows: {
                a!forEach(
                  items: local!data,
                  expression: a!gridRowLayout(
                    id: fv!index,
                    contents: 
                    a!richTextDisplayField(
                      value: a!forEach(
                        items: folder(
                          index(
                            fv!item,
                            "folderId",
                            {}
                          ),
                          "documentChildren"
                        ),/*Getting documents from folder*/
                        expression: {
                          a!richTextItem(
                            text: document(
                              fv!item,
                              "name"
                            ),
                            link: a!documentDownloadLink(
                              document: fv!item
                            )
                          ),
                          a!richTextItem(
                            text: char(10),
                            showWhen: not(
                              fv!isLast
                            )
                          )
                        }
                      )
                    ),
                    a!richTextDisplayField(
                      value: a!forEach(
                        items: folder(
                          index(
                            fv!item,
                            "folderId",
                            {}
                          ),
                          "documentChildren"
                        ),/*Getting documents from folder*/
                        expression: {
                          a!richTextItem(
                            text: document(
                              fv!item,
                              "description"
                            )
                          ),
                          a!richTextItem(
                            text: char(10),
                            showWhen: not(
                              fv!isLast
                            )
                          )
                        }
                      )
                    )
                  )
                )
              },
              selectionSaveInto: {},
              validations: {},
              shadeAlternateRows: true
            )
          )

It's because you are not passing gridRowLayout contents in an array

It's because you are not passing gridRowLayout contents in an array