SSH service allows you to gain access to your system with "shell" command line:

• In Control Panel, section "Terminal and SNMP", check "Enable SSH service" and Apply
• Synology KB: How to login to DSM with root permission via SSH Telnet

Use a SSH client like Putty or Kitty on Windows, or ssh command on Linux to connect to your DSM system refering to its hostname or IP address and providing a username and password.