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. When “Show all filename extensions” is enabled, it takes precedence over each file’s Hide Extension setting. When it is off, you can select a file, choose File > Get Info, and check or uncheck Hide Extension in the Info window to control that file’s extension.
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.
(Featured image created with originals by Patrick Ward on Unsplash and Mateusz Zdrzałek from Pixabay)