The absolute path is a filepath to an application on your Mac that Appium will grab and install before starting the test you are trying to run. Currently the way that people are making this work for getting Mobile Safari open is using an absolute path to a program like SafariLauncher which opens Mobile Safari for you. After that happens you can retrieve the window handle of the webview within Safari and start running normal WebDriver commands.
This workaround is discussed at length here. I can confirm that it works, but the timing of all these components working together is a bit janky, speaking from my testing experience.
For reference, I believe these are the capabilities that I used to get myself up and running.
capabilities.setPlatform(Platform.valueOf("MAC"));
capabilities.setBrowserName("iPhone");
capabilities.setCapability("app", "/path/to/SafariLauncher.app");