"No, he visited a town a little south of throw up."
What Chandler means is that Ross had an "accident" and "pooped" on himself. "Pooped" means that he had a bowel movement...while he was on the roller coaster!
And because I have too much time on my hands, here's an excerpt from that episode:
Chandler: Okay! So, Ross and I are going to Disneyland and we stop at this restaurant for tacos. And when I say restaurant, I mean a guy, a hibachi, and the trunk of his car. So Ross has about 10 tacos. And anyway, we're on Space Mountain and Ross starts to feel a little... iffy.
Monica: Oh, my God. He threw up?
Chandler: No, he visited a little town south of throw-up.
Monica: (Laughing) No!
Chandler: Yeah some of the employees decided to rename the park: "The Crappiest Place on Earth."