The Format Name, also known as the Format Long Name, is additional information about the file format you can find when checking the metadata or exif information of your file. It basically extends the information already given by the file type extension.
File type extensions are abbreviations for the actual, long name of the file format. Thus, the Format Name gives further information about the file without defining the actual file type. For example, we have a file with the extension .mpg and the file type of MPEG-2. The Format Name for this file is defined as "mpeg audio layer 2" or similar.