When Windows has a problem and fails to boot properly, the next time you boot it you will see these options; Safe Mode, Safe Mode with Networking, Safe Mode with Command Prompt.
Safe Mode starts Windows with the bare minimum of drivers and without any “launch at start-up” programs. It gives you a chance to investigate the problem even when you can’t boot fully into Windows. If you have anti-malware software installed, you can try to run a scan in case the problem was caused by malware. Most anti-malware will scan in Safe Mode, though typically real-time protection is disabled.
Safe Mode with Networking is exactly what it says—Safe Mode with additional components enabled to make network connection possible.
Safe Mode with Command Prompt gives you only a Command Prompt to work with; it’s rarely necessary.