Safe mode is a way of starting Windows where it starts only required services. Safe mode allows fixing some problems in windows that cannot have other programs or services running. Sometimes it is what is needed to do to get a virus or other malware cleaned off your machine as the malware may hide it self when it starts when Windows starts. You cannot do networking or run many programs while in safe mode.
In Windows 7 and previous editions you got to safe mode but pressing the F5 key or f8 key repetitively as Windows started and a menu came up and you chose to start in safe mode and Windows restarted in safe mode
In Windows 8 it is a different approach to start in safe mode. The other way no longer works as the start up is too quick. You will pull in the options from the right by moving the cursor to the upper right corner of the screen and the charms bar will come in. Then you will choose Settings, and then choose Restart. Choose Advanced Options. Choose Startup settings and then when it restarts you will have option to choose Safe Mode.
In Windows 10 it is a different approach to start in safe mode. The other way no longer works as the start up is too quick. Click the Windows key (key that looks like the Windows logo) and R. Type msconfig and click ok. Then choose the Boot tab. Then click the button by Safe mode. Click OK and restart and you will restart in Safe Mode.
If you cannot start Windows 8 or 10 you will need the Windows Recovery Drive which you will have to previously have made. So for most people this option will not work as they will not have made it when working correctly but you may be able to get on another machine using same OS and make a recovery drive on a USB thumb/flash drive that works on your machine.