Safe mode is simply where it doesn’t load any 3rd part things etc. Can be useful when troubleshooting. Works as well on pretty much any recent HTC phone (One M7 etc).
It’s rather simple to do. With phone on and active, press power button until you see “phone options menu”. On that you longpress “Power off” and just tap “Restart” on the popup window . To get out of it, just restart phone like normal.