Q:

meaning of the word "enter" in context

Here it goes:

But what about when you are in public and there isn't a bathroom around? Enter the chemical toilet also known as the porta-potty.

I have looked up all the meaning of the word, but still cannot get how someone can enter something.

A:

This comes from stage directions, which were brief - probably because they were originally copied out by hand and people wanted to save effort and paper:

Enter Hamlet
Enter Bob, stage left

It's a direction in a play to say that a character comes onto the stage. It is used idiomatically and metaphorically to talk about something entering the discussion, with the stage being conceptual and metaphorical.

