A hypothetical place where you can say anything, no matter how insulting, outrageous, racist, or insane without fear of getting into trouble, being ridiculed or criticized.
Typically this expression is used to preface something you know will provoke a negative reaction. The term was originally coined by media-personality Glenn Beck in the 2000's.
"...before I say this, I'm going to climb into the safety tree so you can't yell at me. Now that I'm in the safety tree; Are you sure you want to wear those pants? I mean 5 years ago you could do it, but today... ummmm NO."