An open drain output does exactly what it says. Think of an open drain(not electronics). It can only sink people/things. In a controller, an open drain can only sink current. It will not source current. So, if you want to drive an LED, you got to have it sinking current into the pin. When the pin is in input mode, there may not be any obvious difference; however, you need to check the datasheet for the particular device you are using to be sure.