What is this all about? I'm glad you asked!
This is about keeping the Word of Wisdom. This is a code of health that mormons believe Joseph Smith was given by god. It is the 89th section of the Doctrine and Covenants .
So the story goes after one of the men's meetings, Emma Smith was disgusted by all the tobacco all over the floor that they had left for her to clean up and she went to Joseph and complained and he decided to "inquire of the Lord." The result was the Word of Wisdom--which at the time was not commandment, but has since become commandment.
Most mormons are pretty good at the big don'ts--tea, coffee, alcohol, tobacco--but really stink at the smaller ones. the code pretty much suggests that people are meant to be vegetarians; "12Yea, aflesh also of bbeasts and of the fowls of the air, I, the Lord, have ordained for the use of man with thanksgiving; nevertheless they are to be used csparingly;
13And it is pleasing unto me that they should not be aused, only in times of winter, or of cold, or bfamine." so . . . eat it, but only if you have to . . . this is largely ignored by the carnivorous population of the church. There are also restrictions on fats, but you wouldn't know it if you attend an LDS pot luck.
The Word of Wisdom does not specify caffeine and there is some debate as to what substance in coffee and tea god really wanted his children to avoid--caffeine? or tanic acid? (at least I heard a lot of this growing up). This is how come some mormons avoid all caffeine and some guzzle the coke and pepsi.
The word of wisdom is said to be a "principle with a promise"--if you follow it you will live a long and healthy life. Well there is truth to that. Clean living does help you live longer and mormons are famed for their clean living.
It is one of those principles that is cherry-picked though. People do the big parts, and skip the smaller.
Got a question? send it my way at askanexmormon@gmail.com