Joseph has explained to you how an open-drain (or open-collector) output works - it is effectively a switch to ground:
When the transistor is ON, the switch is "closed", ie, the ouput is connected to ground;
When the transistor if OFF, the switch is "open", ie, the ouput is disconnected from ground.

A pull-up resistor will "pull" the output "up" to Vcc when the transistor OFF;
When the transistor is ON, it "pulls" the output "down" to ground.

In the 8051, you turn the P0 transistor OFF by writing a '0' to the Port bit;
In the 8051, you turn the P0 transistor ON by writing a '1' to the Port bit.



In fact, your question suggests that you are not only new to the 8051 in particular, but completely new to any form of electronics - is that right?

You really need to get a book on basic electronics - it is not practical to teach this from first principles by answering one question at a time in a forum like this!

You might like to take a look at the Beginner Tutorials tutorials here:
http://www.iguanalabs.com/maintut.htm

Start with Basic Concepts:
http://www.iguanalabs.com/basicdef.htm