"What is" questions are answered by the WhatIs script under Program.
The script will search for a word definition from Wiktionary, or look up keywords in Freebase. If the keywords are not in Freebase, you will get that error in the logs, and it is nothing to be concerned with. I will try to remove it from being logged as a warning.
If the WhatIs script does not find a result, the bot will answer the question with its best known answer. The "Live and let live." response comes from the bot matching the question "What is the meaning of mobile" with the question "What is the meaning of life", which it knows an answer for "Live and let live.".