A more thorough explanation:

Internal security refers to the measures taken by a country to protect itself from activities that threaten its stability and safety. This includes preventing and responding to acts of terrorism, espionage, sabotage, and other forms of subversion.

Examples of internal security measures include:

• Surveillance of potential threats
• Border control and immigration policies
• Intelligence gathering and analysis
• Law enforcement and counterterrorism operations
• Emergency response planning and training

For instance, a country may monitor the activities of individuals or groups suspected of planning terrorist attacks. It may also implement strict border control measures to prevent the entry of individuals who may pose a threat to national security. In addition, law enforcement agencies may work to disrupt and dismantle terrorist networks through investigations and arrests.

Overall, internal security is an essential aspect of national defense and helps to ensure the safety and well-being of a country's citizens.