On the Mac, nearly every file has an extension, a set of characters after a period that indicates what type of file it is and which app opens it. So, .jpg indicates a Jpeg graphic that opens in Preview by default, .pages denotes a Pages document, .docx identifies a file as belonging to and opening in Microsoft Word, and the extension for all applications is .app. Depending on what you do, how often you exchange files with people on other platforms, and your personal preference, you may wish to see more or fewer extensions. You control that in Finder > Preferences > Advanced, with the “Show all filename extensions” checkbox. Individual files can override the setting, so if an extension isn’t doing what you want, select the file, choose File > Get Info, and check or uncheck the Hide Extension checkbox in the Info window.
Just don’t think it’s a good idea to go turning this feature on simply because you want more control. If you don’t understand how file extensions work you can easily mess things up. If that sentence scares you, you should just leave the feature turned off and let your Mac handle it for you.
If you are an existing customer who needs help with this or if you have other questions, or if you are in San Francisco and interested in becoming a client I invite you to book an appointment with me. Otherwise, you may wish to contact Apple Support or to find a local Apple consultant.