The real UFC 203 weigh-ins happened this morning between 9 and 11 AM ET. CM Punk and Mickey Gall made weight without incident, but the "ceremonial weigh-ins" wrapped up during the 6 PM ET hour, which gave them the chance to do their staredown in front of the crowd.
Gall extended his hand out to CM Punk (real name Phil Brooks), but the former WWE star refused to shake Gall's hand, which was followed by Gall jawing at Punk before Dana White separated them. CM Punk then pumped the crowd up in an enthusiastic manner.
Video of that incident can be seen at the top of the page, courtesy of MMA Fighting.
As for why Punk didn't want to shake Gall's hand, he offered up this response on the FS1 weigh-in show. (Transcription via MMA Fighting)
"I don't know," Punk said. "He was stuttering. He kept repeating, ‘Oh you don't want to shake my hand, you don't want to shake my hand' or something like that. I'm not here to shake anybody's hand. I'm here to punch people in the face."
CM Punk vs. Mickey Gall is the 3rd fight on the PPV main card at UFC 203: Miocic vs. Overeem on Saturday, September 10th in Cleveland, Ohio.