The FLA file format is the "master" document format for Flash projects.
When you create a new Flash file in the Macromedia Flash authoring program, you create an FLA file. This contains all the elements which make up the finished product, including graphics, animation instructions, actionscript code, comments, etc. FLA files can only be opened in Flash (not the Flash Player).
The FLA file is roughly equivalent to the PRPROJ file in Adobe Premiere.
The FLA file does not play or execute. Instead, you export an SWF (or EXE) file which is what end users see. You can preview the resulting SWF file at any time from the FLA file.