Looking up in the conversation

Hello all,
according to the following context extracted form BBC Learning English:
what does 'look up in the conversation' means?!
I couldn't find any equivalent in the dictionary.

Thanks.

Sherry Turkle, American clinical psychologist and professor of the social studies of Science and Technology at Massachusetts Institute of Technology : " I interviewed hundreds of college students and what they talked about was what some of them called 'the rule of three'. And what the rule of three is which is that if you go to dinner with friends, you don't want to look down at your phone until you see that three people, let's say you're six at dinner, are looking up in the conversation. So there's a new etiquette where you don't look down unless three people are looking up kind of to keep a little conversation alive. "