The safe Mode is useful for fixing a variety of problems.  When the computer is running in it, lots of fancier things are disabled.  These fancier things can sometimes cause problems or hide viruses.

Prior to windows 10 (and probably windows 8), you got to it by repeatedly pressing F8 during the boot process then selection Safe Mode when you were prompted for it.

Windows 10 boots so fast that this method does  not work or at lest not as well.

There are a number of ways to get to it but the method that looks the easiest to me is to type "System Configuration" in the "Ask me anything" (this is using Cortana) and select System Configuration desktop app.  Then select the Boot tab and select Safe Boot minimal. Then Apply  then OK. Click on Restart and it will restart in the safe mode.  When done then do the same thing over again but to change it to the regular mode.

Another one is to hold down the Windows Key are press R  This brings up the Run (DOS) window, type in “msconfig” then click on OK.  Then click on the “Boot” tab.  Highlight the version of Windows you are using, then check “Safe Boot”, Apply then OK  Click on Restart and it will restart in the safe mode.  When done then do the same thing over again but to change it to the regular mode.