A file association tells the Mac OS X Finder which kinds of files belong to an application. When you double-click on some file in the Finder, then, based on such a file association, the Finder will open the associated application (and the application will show the file you double-clicked).
The kinds of files that yEd knows (.graphml, .gml, .tgf, etc.)are pre-selected in that install window. Unless you want an application other than yEd to open these kinds of files, there is no reason to deselect any of them.