Extensions don't have any special meaning. They're just what's after the last dot in the file name.
However, like the file name is descriptive of the contents of the file, the extension should (but doesn't have to) be descriptive of the format or purpose of the file. For example, .bin and .dat are common extensions for data in binary; .cfg for configuration files; .ini for INI files; etc.
But nothing keeps you from naming a file "file.thisFileIsUsedForSuchAndSuchWhenSomethingOrOther".