Exactly what it says. It means the collector pin on the output NPN is "open". As in, it's not connected anything else. Specifically, it's not connected to a pull-up transistor and can actively pull the output HI.

That means it can only actively drive the output LO, or disconnect it. That means you have to provide a means to bring the output HI (like a pull-up transistor), or in some cases you don't care if the output is HI as much as you only care if it is conducting current or not(like to drive an LED), in which case you don't worry about a pull-up resistor and just connect the LED to it accordingly.

It means the collector pin on the output NPN is open when in the HI state. As in it's not connected to a pull-up transistor and can actively drive the output HI.

That means it can only actively drive the output LO, or disconnect it. That means you have to provide a means to bring the output HI (like a pull-up transistor), or in some cases you don't care if the output is HI as much as you only care if it is conducting current or not(like to drive an LED), in which case you don't worry about a pull-up resistor and just connect the LED to it accordingly.