This node can be used to read data from an ASCII file or URL location. It reads files where a column is defined by the number of characters and not by a column delimiter.
When you open the node's configuration dialog and provide a filename, the reader creates one column. This column shows only the remaining characters in the preview table and won't be appended to the output table.
The preview shows only the first couple of lines. It could happen then, that the preview appears looking fine, but the execution of the Fixed Width File Reader fails.